The dramatic success and scaling of the Internet was made possible by the core principle of keeping it simple in the middle and smart at the edge (or the end-to-end principle). However, new applications bring new demands, and for many emerging applications, the Internet paradigm presents limitations. For applications in this new generation of Internet services, structured overlay networks offer a powerful framework for deploying specialized protocols that can provide new capabilities beyond what the Internet natively supports by leveraging global state and in-network processing. The structured overlay concept includes three principles: A resilient network architecture, a flexible overlay node software architecture that exploits global state and unlimited programmability, and flow-based processing. We demonstrate the effectiveness of structured overlay networks in supporting today's demanding applications and propose forward-looking ideas for leveraging the framework to develop protocols that push the boundaries of what is possible in terms of performance and resilience.

Existing Byzantine-resilient replication protocols satisfy two standard correctness criteria, safety and liveness, even in the presence of Byzantine faults. The runtime performance of these protocols is most commonly assessed in the absence of processor faults and is usually good in that case. However, faulty processors can significantly degrade the performance of some protocols, limiting their practical utility in adversarial environments. This paper demonstrates the extent of performance degradation possible in some existing protocols that do satisfy liveness and that do perform well absent Byzantine faults. We propose a new performance-oriented correctness criterion that requires a consistent level of performance, even with Byzantine faults. We present a new Byzantine fault-tolerant replication protocol that meets the new correctness criterion and evaluate its performance in fault-free executions and when under attack.

This paper presents the first hierarchical byzantine fault-tolerant replication architecture suitable to systems that span multiple wide-area sites. The architecture confines the effects of any malicious replica to its local site, reduces message complexity of wide-area communication, and allows read-only queries to be performed locally within a site for the price of additional standard hardware. We present proofs that our algorithm provides safety and liveness properties. A prototype implementation is evaluated over several network topologies and is compared with a flat byzantine fault-tolerant approach. The experimental results show considerable improvement over flat byzantine replication algorithms, bringing the performance of byzantine replication closer to existing benign fault-tolerant replication techniques over wide area networks.

Existing Byzantine-resilient replication protocols satisfy two standard correctness criteria, safety and liveness, in the presence of Byzantine faults. In practice, however, faulty processors can, in some protocols, significantly degrade performance by causing the system to make progress at an extremely slow rate. While ldquocorrectrdquo in the traditional sense, systems vulnerable to such performance degradation are of limited practical use in adversarial environments. This paper argues that techniques for mitigating such performance attacks are needed to bridge this ldquopracticality gaprdquo for intrusion-tolerant replication systems. We propose a new performance-oriented correctness criterion, and we show how failure to meet this criterion can lead to performance degradation. We present a new Byzantine replication protocol that achieves the criterion and evaluate its performance in fault-free configurations and when under attack.

Constructing logical machines out of collections of physical machines is a well-known technique for improving the robustness and fault tolerance of distributed systems. We present a new, scalable replication architecture, built upon logical machines specifically designed to perform well in wide-area systems spanning multiple sites. The physical machines in each site implement a logical machine by running a local state machine replication protocol, and a wide-area replication protocol runs among the logical machines. Implementing logical machines via the state machine approach affords free substitution of the fault tolerance method used in each site and in the wide-area replication protocol, allowing one to balance performance and fault tolerance based on perceived risk. We present a new byzantine fault-tolerant protocol that establishes a reliable virtual communication link between logical machines. Our communication protocol is efficient (a necessity in wide-area environments), avoiding the need for redundant message sending during normal-case operation and allowing a logical machine to consume approximately the same wide-area bandwidth as a single physical machine. This dramatically improves the wide-area performance of our system compared to existing logical machine based approaches. We implemented a prototype system and compare its performance and fault tolerance to existing solutions.

This paper presents the first hierarchical Byzantine fault-tolerant replication architecture suitable to systems that span multiple wide area sites. The architecture confines the effects of any malicious replica to its local site, reduces message complexity of wide area communication, and allows read-only queries to be performed locally within a site for the price of additional hardware. A prototype implementation is evaluated over several network topologies and is compared with a flat Byzantine fault-tolerant approach

The phase measurement (PM) method, which makes use of the difference between the input and output of a tunable infinite impulse response (ITR) bandpass filter, is used for adaptive estimation of the fundamental pitch of a signal composed of multiple sinusoids. The estimator has the useful characteristic of ordering the detected frequency components independently of the initial starting values of the section coefficients. In addition to frequency estimation, the amplitude of each component can be extracted from the corresponding bandpass output, which is useful in applications where components envelopes are need for re-synthesis algorithms. The performance of the PM method is compared with that of the recursive least-mean-square method.>