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This thesis examines the issues relating to non-discretionary access controls for decentralized computing systems. Decentralization changes the basic character of a computing system from a set of processes referencing a data base to a set of processes sending and receiving messages. Because messages must be acknowledged, operations that were read-only in a centralized system become read-write operations. As a result, the lattice model of non-discretionary access control, which mediates operations based on read versus read-write considerations, does not allow direct transfer of algorithms from centralized systems to decentralized systems. This thesis develops new mechanisms that comply with the lattice model and provide the necessary functions for effective decentralized computation. Secure

We study contention resolution problem in a multiple-access channel such as the Ethernet...

A communication network is called a radio network if its nodes exchange messages in the following restricted way. First, a send operation performed by a node delivers copies of the same message to all directly reachable nodes. Secondly, a node can successfully receive an incoming message only if exactly one of its neighbors sent a message in that step. It is this semantics of how ports at nodes send and receive messages that defines the networks rather than the fact that only radio waves are used as a medium of communication; but if that is the case then just a single frequency is used. We discuss algorithmic aspects of exchanging information in such networks, concentrating on distributed randomized protocols. Specific problems and solutions depend a lot on the topology of the underlying reachability graph and how much the nodes know about it. In single-hop networks each pair of nodes can communicate directly. This kind of networks is also known as the multiple access channel. Popular

The goal of this paper is to gain deep understanding of how location-dependent propagation latency affects medium access control (MAC) by using ALOHA as a case study. MAC protocols in underwater acoustic networks suffer from latency that is five orders-of-magnitude larger than that in radio networks. Existing work on analyzing MAC throughput in RF networks, where the propagation latency is negligible, generally makes assumptions that render propagation latency irrelevant. As a result, only transmit time is considered as being uncertain in contention-based protocols. We introduce the spatial dimension of uncertainty that is inherent to varying locations of transmitters, resulting in unequal propagation latency to a receiver, where collision occurs. We show through simulation that the benefit of synchronization in slotted ALOHA is lost due to such latency. We propose a modification that adds guard bands to transmission slots to handle spatial uncertainty. We then perform simulation and first order analysis on this modified MAC to find its optimal operating parameters. 1

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When distributed processes contend for a shared resource, we need a good distributed contention resolution protocol, e.g., for multiple-access channels (ALOHA, Ethernet), PRAM emulation, and optical routing. Under a stochastic model of request generation from n synchronous processes, Raghavan & Upfal have shown a protocol which is stable for a positive request rate; their main result is that for every resource request, its expected delay (time to get serviced) is O(log n). Assuming that the initial clock times of the processes are within a known bound of each other, we present a stable protocol, wherein the expected delay for each request is O(1). We derive this by showing an analogous result for an infinite number of processes, assuming that all processes agree on the time. 1 Introduction In scenarios where a set of distributed processes have a single shared resource that can service at most one process per time slot, the main problem is devising a "good" distributed protocol for re...

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We study contention resolution in multiple-access channels such as the Ethernet. Under a stochastic model of continuous packet generation from a set of n processors, we construct a protocol which guarantees constant expected delay for generation rates up to a fixed constant 0 ! 1. Previous protocols which are stable for constant arrival rates do not guarantee constant expected delay. The two protocols that achieved results closest to this are one by Raghavan and Upfal, which only guarantees logarithmic (in n) expected delay, and one by Paterson and Srinivasan, which only guarantees constant expected delay with high probability. (In the latter protocol, there is a non-zero probability that the initial clock synchronization might fail and cause the expected delay to grow unboundedly.) Although those protocols do not guarantee constant expected delay, we have used ideas from them in the construction of our protocol, which does guarantee constant expected delay. We achieve our results usi...

The past decade has seen an explosive growth in wireless products such as cellular phones and this quest for mobility. Since their inception in the late 1970's [FrE80, GfB79], wireless local area networks have been thrust to the forefront of networking alternatives especially when confronted with difficult or impossible obstacles sometimes associated with wired networks such as routing cables and spontaneous ad hoc LANs.