Message logging: pessimistic, optimistic, causal, and optimal

Message-logging protocols are an integral part of a popular technique for implementing processes that can recover from crash failures. All message-logging protocols require that, when recovery is complete, there be no orphan processes, which are surviving processes whose states are inconsistent with the recovered state of a crashed process. We give a precise specification of the consistency property “no orphan processes”. From this specification, we describe how different existing classes of message-logging protocols (namely optimistic, pessimistic, and a class that we call causal) implement this property. We then propose a set of metrics to evaluate the performance of message-logging protocols, and characterize the protocols that are optimal with respect to these metrics. Finally, starting from a protocol that relies on causal delivery order, we show how to derive optimal causal protocols that tolerate f overlapping failures and recoveries for a parameter f (1⩽f⩽n)     

Speculative incoherent cache protocols

Multiprocessing and multithreading are becoming ubiquitous even on single chips. With increasing cache sizes, coherence misses in such systems will account for a larger fraction of all cache misses. As communication latencies increase, this larger fraction of coherence misses will cause significant and increased performance losses. Tuning coherence protocols for specific communication patterns and applications can reduce communication latencies. However, these optimizations increase a protocol's design complexity, making the protocol difficult to verify. A competing approach requires parallel programmers to tune applications to work well with simpler protocols. Speculative execution has successfully improved performance in various scenarios. We propose a new type of load speculation, called coherence decoupling. Coherence decoupling is a microarchitectural mechanism that implements separate protocols for speculative use and for the eventual verification of values. The technique reduces the effect of long communication latencies while mitigating the burdens on the coherence protocol designer and the parallel programmer     

Coordinated opportunistic routing protocol for wireless mesh networks

Opportunistic routing is an emerging research area in Wireless Mesh Networks (WMNs), that exploits the broadcast nature of wireless networks to find the optimal routing solution that maximizes throughput and minimizes packet loss. Opportunistic routing protocols mainly suffer from computational overheads, as most of the protocols try to find the best next forwarding node. In this paper we address the key issue of computational overhead by designing new routing technique without using pre-selected list of potential forwarders. We propose a novel opportunistic routing technique named, Coordinated Opportunistic Routing Protocol for WMNs (CORP-M). We compare CORP-M with well-known protocols, such as AODV, OLSR, and ROMER based on throughput, delivery ratio, and average end-to-end delay. Simulation results show that CORP-M, gives average throughput increase upto 32%, and increase in delivery ratio (from 10% to 20%). We also analyze the performance of CORP-M and ROMER based on various parameters, such as duplicate transmissions and network collisions, by analysis depicts that CORP-M reduces duplicate transmissions upto 70% and network collisions upto 30%.     

A mobility aware protocol synthesis for efficient routing in ad hoc mobile networks

In mobile ad hoc networks (MANETs), the mobility of the nodes is a complicating factor that significantly affects the effectiveness and performance of the routing protocols. Our work builds upon recent results on the effect of node mobility on the performance of available routing strategies (i.e., path-based, using support) and proposes a protocol framework that exploits the usually different mobility rates of the nodes by adapting the routing strategy during execution. We introduce a metric for the relative mobility of the nodes, according to which the nodes are classified into mobility classes. These mobility classes determine, for any pair of origin and destination, the routing technique that best corresponds to their mobility properties. Moreover, special care is taken for nodes remaining almost stationary or moving with high (relative) speeds. Our key design goal is to limit the necessary implementation changes required to incorporate existing routing protocols into our framework. We provide extensive evaluation of the proposed framework, using a well-known simulator (NS2). Our first findings demonstrate that the proposed framework improves, in certain cases, the performance of existing routing protocols.     

Using asynchrony and zero degradation to speed up indulgent consensus protocols

Existing consensus protocols suffer from slowdowns caused by the failures of processes and the mistakes made by the underlying oracles. In this paper, we propose two novel techniques to circumvent such slowdowns in failure-detector-based consensus protocols. The first technique guarantees the Round-Zero-Degradation (RZD) property (an extension of the Zero-Degradation property) in order to avoid the slowdown caused by a failed coordinator process. The second technique, named “Look-Ahead”, helps speed up the execution of the consensus protocol by making use of the messages delivered before their receivers enter the corresponding phases or rounds. The first technique is effective only when the underlying failure detector makes no or few mistakes, while the second technique always works well regardless of the performance of the failure detector. Moreover, Look-Ahead is a general technique and can be applied to consensus protocols based on any kind of oracle. By applying the two proposed techniques, several consensus protocols are developed. The simulation results show that the RZD technique is effective even if the error rate of the failure detector reaches about 15%, while the Look-Ahead technique can always improve the performance in all cases.     

无线传感器网络MAC协议研究进展

在无线传感器网络体系结构中,MAC(medium access control)协议是保证网络高效通信的重要协议.无线传感器网络有着与传统无线网络明显不同的性能特点和技术要求,传统无线网络MAC协议无法应用于传感器网络,各种针对特定传感器网络特点的MAC协议相继提出.归纳无线传感器网络MAC协议的设计原则和分类方法,分析当前典型的各类MAC协议的主要机制,详细比较这些协议的特点、性能差异和应用范围.最后总结无线传感器网络MAC协议的研究现状,指出未来的研究重点.     

The perfectly synchronized round-based model of distributed computing

The perfectly synchronized round-based model provides the powerful abstraction of crash-stop failures with atomic and synchronous message delivery. This abstraction makes distributed programming very easy. We describe a technique to automatically transform protocols devised in the perfectly synchronized round-based model into protocols for the crash, send omission, general omission or Byzantine models. Our transformation is achieved using a round shifting technique with a constant time complexity overhead. The overhead depends on the target model: crashes, send omissions, general omissions or Byzantine failures. Rather surprisingly, we show that no other automatic non-uniform transformation from a weaker model, say from the traditional crash-stop model (with no atomic message delivery), onto an even stronger model than the general-omission one, say the send-omission model, can provide a better time complexity performance in a failure-free execution.     

Ordered shared locks for real-time databases

We propose locking protocols for real-time databases. Our approach has two main motivations: First, locking protocols are widely accepted and used in most database systems. Second, in real-time databases it has been shown that the blocking behavior of transactions in locking protocols results in performance degradation. We use a new relationship between locks called ordered sharing to eliminate blocking that arises in the traditional locking protocols. Ordered, sharing eliminates blocking of read and write operations but may result in delayed termination. Since timeliness and not response time is the crucial factor in real-time databases, our protocols exploit this delay to allow transactions to execute within the slacks of delayed transactions. We compare the performance of the proposed protocols with the two-phase locking protocol for real-time databases. Our experiments indicate that the proposed protocols for real-time databases. Our experiments indicate that the proposed protocols significantly reduce the percentages, of missed deadlines in the system for a variety of workloads.     

Combinatorial design of multi-ring networks with combined routing and flow control

In this paper we present a novel design technique for packet switched networks. The design is based on the construction of multiple virtual rings, which enjoy the one-bridge property: the path between any two nodes is either confined to a single ring or traverses exactly two rings (passing through a single bridge node). Our best designs are constructed by using finite generalized quadrangles of combinatorial design theory. We present novel routing and flow control protocols that capitalize on the one-bridge property of the multi-ring network. Our protocols ensure that (i) no loss due to congestion occurs inside a network, under arbitrary traffic patterns; (ii) all the packets reach their destinations within bounded time with low jitter; and (iii) the bandwidth is allocated fairly and no host is starved. We provide both a theoretical analysis and an extensive simulation-based performance evaluation of our protocols.     

Efficient location of discrepancies in multiple replicated largefiles

We present a new technique for locating corrupted page copies, outdated page copies, and missing page copies in multiple replicated large file copies. We present four communication models and four communication protocols to be used in this technique. These four protocols are classified according to using or not using a coordinator site, groups, and/or a master signature. Whereas, other previous works consider locating only corrupted page copies, our protocols can locate outdated page copies and missing page copies as well as corrupted page copies. In addition, whereas other previous works are based on majority rule to determine the correct copy, our protocols are not. The performances of these protocols are measured in terms of the number of transmissions and the number of transmitted signatures. We compare these performances with one another and, also, compare them to the performances of other previous works     

Scalable RFID security protocols supporting tag ownership transfer

We identify privacy, security and performance requirements for radio frequency identification (RFID) protocols, as well as additional functional requirements such as tag ownership transfer. Many previously proposed protocols suffer from scalability issues because they require a linear search to identify or authenticate a tag. In support of scalability, some RFID protocols, however, only require constant time for tag identification, but, unfortunately, all previously proposed schemes of this type have serious shortcomings. We propose a novel scalable RFID authentication protocol based on the scheme presented in Song and Mitchell (2009) [1], that takes constant time to authenticate a tag. We also propose secret update protocols for tag ownership and authorisation transfer. The proposed protocols possess the identified privacy, security and performance properties and meet the requirements for secure ownership transfer identified here.     

Software Distributed Shared Memory: a VIA‐based implementation and comparison of sequential consistency with home‐based lazy release consistency

A Distributed Shared Memory (DSM) system provides a distributed application with a shared virtual address space. This article proposes a design for implementing the DSM communication layer on top of the Virtual Interface Architecture (VIA), an industry standard for user‐level networking protocols on high‐speed clusters. User‐level communication protocols operate in user mode, thus removing the operating system kernel's overhead from the critical communication pass, and significantly diminishing communication overhead as a result. We analyze VIA's facilities and limitations in order to ascertain which implementation trade‐offs can be best applied to our development of an efficient communication substrate optimized for DSM requirements. We then implement a multithreaded version of the Home‐based Lazy Release Consistency (HLRC) protocol on top of this substrate. In addition, we compare the performance of this HLRC protocol with that of the Sequential Consistency (SC) protocol in which a Multi View (MV) memory mapping technique was used. This technique enables a fine‐grained access to shared memory, while still relying on the virtual memory hardware to track memory accesses. We perform an ‘apple‐to‐apple’ comparison on the same testbed environment and benchmark suite, and investigate the effectiveness and scalability of both protocols. Copyright © 2005 John Wiley & Sons, Ltd.     

Target bandwidth sharing using endhost measures

Recent congestion control protocols such as XCP and RCP achieve fair bandwidth sharing, high utilization, small queue sizes and nearly zero packet loss by implementing an explicit bandwidth share mechanism in the network routers. This paper develops new quantitative techniques for achieving the same results using only end-host measures. We develop new methods of computing bottleneck link characteristics, a new technique for sharing bandwidth fairly with Reno flows, and a new approach for rapidly converging to bandwidth share. A new transport protocol, TCP-Madison, that employs the new bandwidth sharing techniques is also defined in the paper. Experiments comparing TCP-Madison with FAST TCP, BIC-TCP and TCP-Reno over hundreds of PlanetLab and other live Internet paths show that the new protocol achieves the stated bandwidth sharing properties, is easily configured for near-optimal performance over all paths, and significantly outperforms the previous protocols.     

基于FPGA的并行多发可编程解析器

传统的报文解析器解析的协议类型和协议层次固定,缺乏对新网络协议的支撑,限制了网络设备的可编程性。抽象出形式化的解析流程,并基于FPGA实现协议无关的可编程解析器,对新协议的支撑无需更改硬件,仅需要重新映射解析图。基于该机制,引入一系列优化技术,克服了包解析固有的串行性,节约了存储资源,为实现高速的可编程报文解析提供了有效的解决方案。基于通用多核和高性能FPGA实验平台,进行了硬件代价和性能的评估。实验结果表明,采用可编程解析器能大幅提升报文解析性能,实现了通用网络协议及潜在的网络协议快速的解析,可有效地支持快速的定制网络协议发展。     

Robust self-keying mobile ad hoc networks

Pairwise key establishment in mobile ad hoc networks allows any pair of nodes to agree upon a shared key. This is an important security service needed to secure routing protocols, and in general to facilitate secure communication among the nodes of the network.We present two self-keying mechanisms for pairwise key establishment in mobile ad hoc networks which do not require any centralized support. The mechanisms are built using the well-known technique of threshold secret sharing, and are robust and secure against a collusion of up to a certain number of nodes. We evaluate and compare the performance of both the mechanisms in terms of the node admission and pairwise key establishment.     

并行数据库实时多版本并发控制协议性能研究

随着对实时数据库事务处理性能要求的不断提高 ,将并行数据库和实时数据库相结合的并行实时数据库将成为新的发展方向 .然而通常的实时多版本并发控制协议不适合并行数据库的无共享结构 .比较了几种并发控制协议在无共享结构下的性能 ,该文提出了一种并行实时多版本并发控制协议 .它能有效地减少事务的重启 ,降低事务的错过率 .在长短事务混合的情况下 ,它的性能比其它的并发控制协议更好 .同时 ,它具有更好的可扩展性 .作者利用仿真模型对该协议进行了性能测试 ,与其它协议进行了比较 ,并分析了实验结果 ,仿真实验结果表明该协议性能良好     

Universal Continuous Routing Strategies

We analyze universal routing protocols, that is, protocols that can be used for any communication pattern in any network, under a stochastic model of continuous message generation. In particular, we present two universal protocols, a store-and-forward and a wormhole routing protocol, and characterize their performance by the following three parameters: the maximum message generation rate for which the protocol is stable, the expected delay of a message from generation to service, and the time the protocol needs to recover from worst-case scenarios. Both protocols yield significant performance improvements over all previously known continuous routing protocols. In addition, we present adaptations of our protocols to continuous routing in node-symmetric networks, butterflies, and meshes. Received October 1996, and in final form April 1997.     

Comparative Evaluation of Latency-Tolerating and -Reducing Techniques for Hardware-Only and Software-Only Directory Protocols

We study in this paper how effective latency-tolerating and -reducing techniques are at cutting the memory access times for shared-memory multiprocessors with directory cache protocols managed by hardware and software. A critical issue for the relative efficiency is how many protocol operations such techniques trigger. This paper presents a framework that makes it possible to reason about the expected relative efficiency of a latency-tolerating or -reducing technique by focusing on whether the technique increases, decreases, or does not change the number of protocol operations at the memory module. Since software-only directory protocols handle these operations in software they will perform relatively worse unless the technique reduces the number of protocol operations. Our experimental results from detailed architectural simulations driven by six applications from the SPLASH-2 parallel program suite confirm this expectation. We find that while prefetching performs relatively worse on software-only directory protocols due to useless prefetches, there are examples of protocol optimizations, e.g., optimizations for migratory data, that do relatively better on software-only directory protocols. Overall, this study shows that latency-tolerating techniques must be more carefully selected for software-centric than for hardware-centric implementations of distributed shared-memory systems.