An ad hoc network is a collection of wireless mobile hosts forming a temporary network without the aid of any established infrastructure or centralized administration. In such an environment, it may be necessary for one mobile host to enlist the aid of other hosts in forwarding a packet to its destination, due to the limited range of each mobile host’s wireless transmissions. This paper presents a protocol for routing in ad hoc networks that uses dynamic source routing. The protocol adapts quickly to routing changes when host movement is frequent, yet requires little or no overhead during periods in which hosts move less frequently. Based on results from a packet-level simulation of mobile hosts operating in an ad hoc network, the protocol performs well over a variety of environmental conditions such as host density and movement rates. For all but the highest rates of host movement simulated, the overhead of the protocol is quite low, falling to just 1 % of total data packets transmitted for moderate movement rates in a network of 24 mobile hosts. In all cases, the difference in length between the routes used and the optimal route lengths is negligible, and in most cases, route lengths are on average within a factor of 1.01 of optimal. 1.

Overcast is an application-level multicasting system that can be incrementally deployed using today's Internet infrastructure. These properties stem from Overcast's implementation as an overlay network. An overlay network consists of a collection of nodes placed at strategic locations in an existing network fabric. These nodes implement a network abstraction on top of the network provided by the underlying substrate network.

The Dynamic Source Routing protocol (DSR) is a simple and efficient routing protocol designed specifically for use in multi-hop wireless ad hoc networks of mobile nodes. DSR allows the network to be completely self-organizing and self-configuring, without the need for any existing network infrastructure or administration. The protocol is composed of the two mechanisms of Route Discovery and Route Maintenance, which work together to allow nodes to discover and maintain source routes to arbitrary destinations in the ad hoc network. The use of source routing allows packet routing to be trivially loop-free, avoids the need for up-to-date routing information in the intermediate nodes through which packets are forwarded, and allows nodes forwarding or overhearing packets to cache the routing information in them for their own future use. All aspects of the protocol operate entirely on-demand, allowing the routing packet overhead of DSR to scale automatically to only that needed to react to changes in the routes currently in use. We have evaluated the operation of DSR through detailed simulation on a variety of movement and communication patterns, and through implementation and significant experimentation in a physical outdoor ad hoc networking testbed we have constructed in Pittsburgh, and have demonstrated the excellent performance of the protocol. In this chapter, we describe the design of DSR and provide a summary of some of our simulation and testbed implementation results for the protocol. 1

In systems based on the client-server model, a single server may serve many clients and the heavy load on the server may cause the response time to be adversely affected. In such circumstances, replicating data or servers may improve performance. Replication may also improve the availability of information when processors crash or the network partitions. Existing replication methods are often needlessly expensive. They sometimes use pointto -point communication when multicast communication is available; they typically pay the full price of end-to-end acknowledgments for all of the participants for every update; they may claim locks, and therefore, may be vulnerable to faults that can unnecessarily block the system for long periods of time. This thesis presents a new architecture and algorithms for replication over a partitioned network. The architecture is structured into two layers: a replication server and a group communication layer. Each of the replication servers maintains a priva...

A process is an operating system abstraction representing an instance of a running computer program. Process migration is the act of transferring a process between two machines during its execution. Several implementations

The Amoeba group communication system has two unique aspects: (1) it uses a sequencer-based protocol with negative acknowledgements for achieving a total order on all group messages; and (2) users choose the degree of fault tolerance they desire. This paper reports on our design decisions in retrospect, the performance of the Amoeba group system, and our experiences using the system. We conclude that sequencer-based group protocols achieve high performance (comparable to Amoeba's fast remote procedure call implementation), that the scalability of our sequencer-based protocols is limited by message processing time, and that the flexibility and modularity of user-level implementations of protocols is likely to outweigh the potential performance loss.

With the advent of large-scale parallel computing systems, making parallel programs fault-tolerant becomes an important problem, because the probability of a failure increases with the number of processors. In this paper, we describe a very simple scheme for rendering a class of parallel Orca programs fault-tolerant. Also, we discuss our experience with implementing this scheme on Amoeba. Our approach works for parallel applications that are not interactive. The approach is based on making a globally consistent checkpoint from time to time and rolling back to the last checkpoint when a processor fails. Making a consistent global checkpoint is easy in Orca, because its implementation is based on reliable broadcast. The advantages of our approach are its simplicity, ease of implementation, low overhead, and transparency to the Orca programmer. 1.

... decade has led to an increasingly nomadic computing lifestyle. A computer is no longer an immobile, gargantuan machine that remains in one place for the lifetime of its operation. Today's personal computing devices are portable, and Internet access is becoming ubiquitous. A well-traveled laptop user might use half a dozen different networks throughout the course of a day: a cable modem from home, wide-area wireless on the commute, wired Ethernet at the office, a Bluetooth network in the car, and a wireless, local-area network at the airport or the neighborhood coffee shop. Mobile host

Group communication is an important paradigm for building distributed applications. This paper discusses a fault-tolerant distributed directory service based on group communication, and compares it with the previous design and implementation based on remote procedure call. The group directory service uses an active replication scheme and, when triplicated, can handle 627 lookup operations per second and 88 update operations per second (using nonvolatile RAM). This performance is better than the performance for the RPC implementation and it is even better than the performance for directory operations under SunOS, which does not provide any fault tolerance at all. The paper concludes that the implementation using group communication is simpler and has better performance than the one based on remote procedure call, supporting the claim that a distributed operating system should provide both remote procedure call and group communication.

The design of a process migration mechanism for the Amoeba distributed operating system is described. The primary motivation for this implementation is to carry out experimental and realistic studies of load balancing algorithms for a distributed operating system. Our aim has been the implementation of a mechanism which is general, efficient and fully transparent, and which is reliable in the presence of network and processor failures. 1