Mustaque Ahamad

Securing context-aware applications using environment roles

In the future, a largely invisible and ubiquitous computing infrastructure will assist people with a variety of activities in the home and at work. The applications that will be deployed in such systems will create and manipulate private information and will provide access to a variety of other resources. Securing such applications is challenging for a number of reasons. Unlike traditional systems where access control has been explored, access decisions may depend on the context in which requests are made. We show how the well-developed notion of roles can be used to capture security-relevant context of the environment in which access requests are made. By introducing environment roles, we create a uniform access control framework that can be used to secure context-aware applications. We also present a security architecture that supports security policies that make use of environment roles to control access to resources.

Causal memory: definitions, implementation, and programming

SummaryThe abstraction of a shared memory is of growing importance in distributed computing systems. Traditional memory consistency ensures that all processes agree on a common order of all operations on memory. Unfortunately, providing these guarantees entails access latencies that prevent scaling to large systems. This paper weakens such guarantees by definingcausal memory, an abstraction that ensures that processes in a system agree on the relative ordering of operations that arecausally related. Because causal memory isweakly consistent, it admits more executions, and hence more concurrency, than either atomic or sequentially consistent memories. This paper provides a formal definition of causal memory and gives an implementation for message-passing systems. In addition, it describes a practical class of programs that, if developed for a strongly consistent memory, run correctly with causal memory.

Incentives in BitTorrent induce free riding

We investigate the incentive mechanism of BitTorrent, which is a peer-to-peer file distribution system. As downloaders in BitTorrent are faced with the conflict between the eagerness to download and the unwillingness to upload, we relate this problem to the iterated prisoners dilemma, which suggests guidelines to design a good incentive mechanism. Based on these guidelines, we propose a new, simple incentive mechanism. Our analysis and the experimental results using PlanetLab show that the original incentive mechanism of BitTorrent can induce free riding because it is not effective in rewarding and punishing downloaders properly. In contrast, a new mechanism proposed by us is shown to be more robust against free riders.

The Clouds distributed operating system

The authors discuss a paradigm for structuring distributed operating systems, the potential and implications this paradigm has for users, and research directions for the future. They describe Clouds, a general-purpose operating system for distributed environments. It is based on an object-thread model adapted from object-oriented programming.<<ETX>>

The grid protocol: a high performance scheme for maintaining replicated data

A protocol for maintaining replicated data that can provide both high data availability and low response time is presented. Existing protocols are designed primarily to achieve high availability by updating a large fraction of the copies, which provides some (although not significant) load sharing. In the new protocol, transaction processing is shared effectively among nodes storing copies of the data, and both the response time experienced by transactions and the system throughput are improved significantly. Also presented is an analysis of the availability of the new protocol and simulation is used to study the effect of load sharing on the response time of transactions. The new protocol is also compared with a voting-based scheme.<<ETX>>

Slow memory: weakening consistency to enhance concurrency in distributed shared memories

The use of weakly consistent memories in distributed shared memory systems to combat unacceptable network delay and to allow such systems to scale is proposed. Proposed memory correctness conditions are surveyed, and how they are related by a weakness hierarchy is demonstrated. Multiversion and messaging interpretations of memory are introduced as means of systematically exploring the space of possible memories. Slow memory is presented as a memory that allows the effects of writes to propagate slowly through the system, eliminating the need for costly consistency maintenance protocols that limit concurrency. Slow memory processes a valuable locality property and supports a reduction from traditional atomic memory. Thus slow memory is as expressive as atomic memory. This expressiveness is demonstrated by two exclusion algorithms and a solution to M.J. Fischer and A. Michaels (1982) dictionary problem on slow memory.<<ETX>>

The power of processor consistency

Shared memories that provide weaker consistency guarantees than the traditional sequentially consistent or atomic memories have been claimed to provide the key to building scalable systems. One influential memory model, processor considency, has been cited widely in the literature but, due to the lack of a precise and formal definition, contradictory claims have been made regarding its power. We use a formal model to give two distinct definitions of processors consistency: one corresponding to Goodman’s original proposal and the other corresponding that given by the implementors of the DASH system. These definitions are non-operational and can be easily related to other types of memories. To illustrate the power of processor consistency, we exhibit a non-cooperative solution to the mutual exclusion problem that is correct with processor consistency. As a contrast, we show that Lamport’s Bakery algorithm is not correct with processor consistency.

Performance characterization of quorum-consensus algorithms for replicated data

The authors develop a model and define performance measures for a replicated data system that makes use of a quorum-consensus algorithm to maintain consistency. They consider two measures: the proportion of successfully completed transactions in systems where a transaction aborts if data is not available, and the mean response time in systems where a transaction waits until data becomes available. Based on the model, the authors show that for some quorum assignment there is an optimal degree of replication beyond which performance degrades. There exist other quorum assignments which have no optimal degree of replication. The authors also derive optimal read and write quorums which maximize the proportion of successful transactions. >

Timed consistency for shared distributed objects

Ordering and time are two different aspects of consistency of shared objects in a distributed system. One avoids conflicts between operations, the other addresses how quickly the effects of an operation are perceived by the rest of the system. Consistency models such as sequential consistency and causal consistency do not consider the particular time at which an operation is executed to establish a valid order among all the operations of a computation. Timed consistency models require that if a write operation is executed at time t, it must be visible to all nodes by time t +Δ. Timed consistency generalizes several existing consistency criteria and it is well suited for interactive and collaborative applications, where the action of one user must be seen by others in a timely fashion.

Implementing and programming causal distributed shared memory

A simple owner protocol for implementing a causal distributed shared memory (DSM) is presented, and it is argued that this implementation is more efficient than comparable coherent DSM implementations. Moreover, it is shown that writing programs for causal memory is no more difficult than writing programs for atomic shared memory.<<ETX>>