Given a ring of n processors it is required to design the processors such that they will be able to choose a leader (a uniquely designated processor) by sending messages along the ring. If the processors are indistinguishable then there exists no deterministic algorithm to solve the problem. To overcome this difficulty, probabilistic algorithms are proposed. The algorithms may run forever but they terminate within finite time on the average. For the synchronous case several algorithms are presented: The simplest requires, on the average, the transmission of no more than 2.442n bits and O (n) time. More sophisticated algorithms trade time for communication complexity. If the processors work asynchronously then on the average O (nlogn) bits are transmitted. In the above cases the size of the ring was assumed to be known to all the processors. If the size is not known then finding it may be done only with high probability: any algorithm may yield incorrect results (with nonzero probabilit...

The theory developed in Part I is used to state the mutual exclusion problem and several additional fairness and failure-tolerance requirements. Four "distributed " N-process solutions are given, ranging from a solution requiring only one communication bit per process that permits individual starvation, to one requiring about N ! communication bits per process that satisfies every reasonable fairness and failure-tolerance requirement that we can conceive of. Contents 1 Introduction 3 2 The Problem 4 2.1 Basic Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Fairness Requirements . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Premature Termination . . . . . . . . . . . . . . . . . . . . . 8 2.4 Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3 The Solutions 14 3.1 The Mutual Exclusion Protocol . . . . . . . . . . . . . . . . . 15 3.2 The One-Bit Solution . . . . . . . . . . . . . . . . . . . . . . 17 3.3 A Digression . . . . . . . . . . . ...

We survey results from distributed computing that show tasks to be impossible, either outright or within given resource bounds, in various models. The parameters of the models considered include synchrony, fault-tolerance, different communication media, and randomization. The resource bounds refer to time, space and message complexity. These results are useful in understanding the inherent difficulty of individual problems and in studying the power of different models of distributed computing.

This paper considers distributed protocols for assigning IDs.

The physical user interface is an increasingly significant factor limiting the effectiveness of our interactions with and through technology. This thesis introduces Electric Field Imaging, a new physical channel and inference framework for machine perception of human action. Though electric field sensing is an important sensory modality for several species of fish, it has not been seriously explored as a channel for machine perception. Technological applications of field sensing, from the Theremin to the capacitive elevator button, have been limited to simple proximity detection tasks. This thesis presents a solution to the inverse problem of inferring geometrical information about the configuration and motion of the human body from electric field measurements. It also presents simple, inexpensive hardware and signal processing techniques for making the field measurements, and several new applications of electric field sensing. The signal

Modeling frameworks such as Probabilistic I/O Automata (PIOA) and Markov Decision Processes permit both probabilistic and nondeterministic choices. In order to use such frameworks to express claims about probabilities of events, one needs mechanisms for resolving nondeterministic choices. For PIOAs, nondeterministic choices have traditionally been resolved by schedulers that have perfect information about the past execution. However, such schedulers are too powerful for certain settings, such as cryptographic protocol analysis, where information must sometimes be hidden. Here, we propose a new, less powerful nondeterminism-resolution mechanism for PIOAs, consisting of tasks and local schedulers. Tasks are equivalence classes of system actions that are scheduled by oblivious, global task sequences. Local schedulers resolve nondeterminism within system components, based on local information only. The resulting task-PIOA framework yields simple notions of external behavior and implementation, and supports simple compositionality results. We also define a new kind of simulation relation, and show it to be sound for proving implementation. We illustrate the potential of the task-PIOA framework by outlining its use in verifying an Oblivious Transfer protocol.

Implementations of inter-process communication and synchronization in distributed systems usually rely on the existence of unique ids for the processes. We consider the problem of generating such ids for identical processes in a shared-variable system. A randomized protocol that assigns distinct ids to the processes within an expected polynomial number of rounds using a polynomial number of boolean atomic variables is presented.

Abstract. The paper studies automatic verification of liveness properties with probability 1 over parameterized programs that include probabilistic transitions, and proposes two novel approaches to the problem. The first approach is based on a Planner that occasionally determines the outcome of a finite sequence of “random” choices, while the other random choices are performed non-deterministically. Using a Planner, a probabilistic protocol can be treated just like a non-probabilistic one and verified as such. The second approach is based on γ-fairness, a notion of fairness that is sound and complete for verifying simple temporal properties (whose only temporal operators are and) over finite-state systems. The paper presents a symbolic model checker based on γ-fairness. We then show how the network invariant approach can be adapted to accommodate probabilistic protocols. The utility of the Planner approach is demonstrated on a probabilistic mutual exclusion protocol. The utility of the approach of γ-fairness with network invariants is demonstrated on Lehman and Rabin’s Courteous Philosophers algorithm. 1

Identifying what problems can be solved in a given distributed system is a central question in distributed computing. In this series of works, we study this question in the context of asynchronous fault tolerant systems that can execute consensus. These systems can be those executing deterministic protocols with access to a consensus routine or those running randomized error-free protocols. A previous work handled the class of distributed decision tasks. In these tasks, each processor receives one local input and has to respond with one local output.

This paper discusses a group coordination architecture to support Internet-wide distributed collaboration in the context of legacy Internet protocols. Group coordination in distributed systems and multimedia systems has many faces manifested in a variety of user interfaces and network protocols. To date, no standardized methodology for engineering group coordination protocols exists. We perceive coordination as the third complementary component in the trinity of group-communication services, next to membership and dissemination. With the current surge in e-commerce and Web-leveraged information exchange among users, the need for systems offering better telepresence and interaction capabilities becomes tangible. Services to support distributed group interaction at (near) real-time, with userspecified Quality-of-Service, and at Internet scope are of particular interest in this mosaic of telepresence and remote collaboration. In this paper, we propose a general group coordination architecture for heterogeneous networks, as a framework to leverage the rapid development of grouporiented distributed collaborative applications in the Internet, for example for distance education, distributed scientific simulation or visualization, and similar applications. 1.