移动分布式实时事务实时原子提交

形式地给出了移动分布式实时事务实时原子提交协议的定义,在此基础上提出了适合于移动分布式实时事务的实时原子提交协议：一阶段实时原子提交协议(1PRACP)。1PRACP通过参与者与协调者的一次消息交换,在一个阶段完成移动分布式实时事务提交活动;结合超时恢复处理协议,1PRACP能避免由于站点故障或网络通信链路故障而导致的阻塞。对1PRACP进行了性能比较和评测,显示了它在各方面的优越性。     

A cross-layer atomic commit protocol implementation for transaction processing in mobile ad-hoc networks

Transaction processing leads to new challenges in mobile ad-hoc networks, which, in comparison to fixed-wired networks, suffer from problems like node disconnection, message loss, and frequently appearing network partitioning. As the atomic commit protocol is that part of transaction processing in which failures can lead to the most serious data blocking, we have developed a robust and failure-tolerant distributed cross-layer atomic commit protocol called CLCP that uses multiple coordinators. In order to reduce the number of both, failures and messages, our protocol makes use of acknowledgement messages for piggybacking information. We have evaluated our protocol in mobile ad-hoc networks by using several mobility models (i.e. Random Waypoint, Manhattan, and Attraction Point), and compared CLCP with other atomic commit protocols, i.e. 2PC and Paxos Commit, each implemented in 3 versions, i.e. without sending message acknowledgements, with a Relay Routing technique, and with Nearest Forward Progress Routing. Special to our simulation environment is the use of the quasi-unit-disc model, which assumes a non-binary message reception probability that captures real-world behavior much better than the classical unit-disc-model, often used in theory. Using the quasi-unit-disc model, our evaluation shows the following results. CLCP and “2PC without acknowledgement messages” have a significantly lower energy consumption than the other protocols, and CLCP is able to commit significantly more distributed transactions than all the other atomic commit protocols for each of the mobility models.     

Complexity of distributed commit protocols

Summary Implementation of atomic actions in a distributed system in the presence of fail-stop failures is investigated. Worst case time and message complexities of the protocols realizing this are studied on complete graphs, rings, trees, and arbitrary graphs. Two modes of communication are considered — point-to-point and broadcast. Individual lower and upper bounds on time and messages are presented and the simultaneous achievability of the optimum message and time bounds is shown impossible in all the interesting cases.     

Lightweight security for mobile commerce transactions

This paper describes a lightweight security mechanism for protecting electronic transactions conducted over the mobile platform. In a typical mobile computing environment, one or more of the transacting parties are based on some wireless handheld devices. Electronic transactions conducted over the mobile platform are gaining popularity and it is widely accepted that mobile computing is a natural extension of the wired Internet computing world. However, security over the mobile platform is more critical due to the open nature of wireless networks. Furthermore, security is more difficult to implement on the mobile platform because of the resource limitation of mobile handheld devices. Therefore, security mechanisms for protecting traditional computer communications need to be revisited so as to ensure that electronic transactions involving mobile devices can be secured and implemented in an effective manner. This research is part of our effort in designing security infrastructure for electronic commerce systems, which extend from the wired to the wireless Internet. A lightweight mechanism was designed to meet the security needs in face of the resource constraints. The proposed mechanism is proven to be practical in real deployment environment.     

Modeling and performance evaluation of transport protocols for firewall control

Firewalls are a crucial building block for securing IP networks. The usage of out-of-band signaling protocols such as SIP for IP telephony and multimedia applications requires a dynamic control of these firewalls and imposes several challenges. Recently, several firewall control architectures and protocols have been developed. The main focus of this paper is the simple middlebox configuration protocol (SIMCO), which is a new transaction-based firewall control protocol. Due to the impact on call setup delays, firewall signaling requires small end-to-end delays and thus mandates a careful choice of the transport protocol. Therefore, this paper studies SCTP, TCP and UDP-based transport for SIMCO and compares different configurations that allow to optimize the performance. We present an analytical model to quantify the impact of head-of-line blocking in SCTP and TCP and verify it with measurements. Both the model and measurements reveal that SCTP can significantly reduce the SIMCO response times by leveraging transmission over multiple parallel streams. While already a few SCTP streams can almost completely avoid head-of-line blocking, our results show that TCP- and UDP-based transport may suffer from significantly larger delays.     

An atomic commit protocol for gigabit-networked distributed database systems

In the near future, different database sites will be interconnected via gigabit networks, forming a very powerful distributed database system. In such an environment, the propagation latency will be the dominant component of the overall communication cost while the migration of large amounts of data will not pose a problem. Furthermore, computer systems are expected to become even more reliable than today’s systems with long mean time between failures and short mean time to repair. In this paper, we present implicit yes-vote (IYV), a one-phase atomic commit protocol, that exploits these new domain characteristics to minimize the cost of distributed transaction commitment. IYV eliminates the explicit voting phase of the two-phase commit protocol, hence reducing the number of sequential phases of message passing during normal processing. In the case of a participant’s site failure, IYV supports the option of forward recovery by enabling partially executed transactions that are still active in the system to resume their execution when the failed participant is recovered.     

Mean-field framework for performance evaluation of push-pull gossip protocols

Gossip protocols are designed to operate in very large, decentralised networks. A node in such a network bases its decision to interact (gossip) with another node on its partial view of the global system. Because of the size of these networks, analysis of gossip protocols is mostly done using simulations, but these tend to be expensive in computation time and memory consumption.We employ mean-field analysis techniques for the evaluation of gossip protocols. Nodes in the network are represented by small identical stochastic processes. Joining all nodes would result in an enormous stochastic process. If the number of nodes goes to infinity, however, mean-field analysis allows us to replace this intractably large stochastic process by a small deterministic process. This process approximates the behaviour of very large gossip networks, and can be evaluated using simple matrix-vector multiplications.     

Analysis and performance evaluation of HYPERchannel access protocols

HYPERchannel [1,3,5,7] is a local network configuration with random access to the global transmission medium and with ‘Carrier Sense’ techniques. Conflicts are resolved by a fixed priority scheduling of retransmissions.This paper presents a performance evaluation of HYPERchannel access protocols for different types of user systems. Analytical results and asymptotic relations are obtained for the throughput of HYPERchannel under different load characteristics.The fixed priority rule for retransmissions discriminates against low priority users; a modification of the access scheme (‘Fair HYPERchannel’) is proposed which compensates for this undesirable effect. The performance of this new protocol version is also evaluated.In a final section, the access schemes of HYPERchannel are compared with similar access protocols.     

On the challenge and design of transport protocols for MMORPGs

Although MMORPGs are becoming increasingly popular as well as a highly profitable Internet business, there is still a fundamental design question: Which transport protocol should be used—TCP, UDP, or some other protocol? In this paper, we first evaluate whether TCP is suitable for MMORPGs, and then propose some novel transport strategies for this genre of games. Our analysis of a trace collected from a TCP-based MMORPG called ShenZhou Online indicates that TCP is unwieldy and inappropriate for MMORPGs. We find that the degraded network performance problems are due to the following characteristics of MMORPG traffic: 1) tiny packets, 2) a low packet rate, 3) application-limited traffic generation, and 4) bi-directional traffic. Since not all game packets require reliable transmission or in-order delivery, transmitting all packets with a strict delivery guarantee causes high delays and delay jitters. Therefore, our proposed transport strategies assign game packets with appropriate levels of transmission guarantee depending on the requirements of the packets’ contents. To compare the performance of our approach with that of existing transport protocols, we conduct network simulations with a real-life game trace from Angel’s Love. The results demonstrate that our strategies significantly reduce the end-to-end delay and delay jitter of packet delivery. Finally, we show that our strategies effectively raise satisfaction levels of the game players.     

Broadcast-efficient protocols for mobile radio networks

The main contribution of this work is to present elegant broadcast-efficient protocols for permutation routing, ranking, and sorting on single-hop Mobile Radio Networks with p stations and k radio channels, denoted by MRN(p,k). Clearly, any protocol performing these tasks on n items must perform ⁿ/_k broadcast rounds because each item must be broadcast at least once. We begin by presenting an optimal off-line permutation routing protocol using ⁿ /_k broadcast rounds for arbitrary k, p, and n. Further, we show that optimal on-line routing can be performed in ⁿ/ _k broadcast rounds, provided that either k=1 or p=n. We then go on to develop an online routing protocol that takes 2ⁿ/ _k+k-1 broadcast rounds on the MRN(p,k), whenever k⩽√^p/₂. Using these routing protocols as basic building blocks, we develop a ranking protocol that takes 2ⁿ /_k+o(ⁿ/_k) broadcast rounds as well as a sorting protocol that takes 3ⁿ/_k+o(ⁿ/_k) broadcast rounds, provided that k ϵ o(√n) and p=n. Finally, we develop a ranking protocol that takes 3ⁿ/_k+o(ⁿ/ _k) broadcast rounds, as well as a sorting protocol that takes 4ⁿ/_k+o(ⁿ/_k) broadcast rounds on the MRN(p,k), provided that k⩽√^p/₂ and p ϵ o(n). Featuring very low proportionality constants, our protocols offer a vast improvement over the state of the art     

A new model for context-aware transactions in mobile services

With the ubiquity of handheld devices (such as smart phones and PDAs) and the availability of a wide range of mobile services (such as mobile banking, road traffic updates, and weather forecast), people can nowadays access information and conduct online transactions virtually anywhere and anytime. In such flexible, dynamic but less reliable environment, transaction management technology is believed to provide service reliability and data consistency. Indeed, in mobile and ubiquitous environments where devices as well as services can seamlessly join and leave the ubiquitous network; transaction management can be very helpful during the recovery of services from failure. Current transaction models and commit protocols do not take into account context information. However, in mobile environments, it is imperative to consider context information in the commit of a transaction—i.e., a transaction can be successfully completed if it meets the required context. In this paper, we propose a new model for context-aware transactions and their performance management in mobile environments. Unlike conventional transactions, context-aware transactions adapt to the required context. By context, we mean the service’s context as well as the users’ context that includes users’ needs and preferences. This paper designs and develops the proposed transaction model and evaluates its performance in terms of time and message complexities as well as transaction’s throughput.     

Deployment and performance evaluation of mobile multicoupon solutions

Mobile commerce (m-commerce) represents an important area of business with a huge potential revenue for merchants and great opportunities for customers to achieve better offers. One of the topics within m-commerce that needs important improvements on usability and efficiency is electronic coupon systems. This is because solutions for electronic coupons are focused on their theoretical definition, overlooking both their performance evaluation and their viability analysis. In this paper, we provide an evaluation framework to test multicoupon solutions. Under this framework, we evaluate two particular multicoupon schemes, obtaining interesting performance results, such as the fact that cryptographic operations are not always the most costly processes, as usually claimed by authors of multicoupon schemes. We show that message encoding can help to improve the efficiency of multicoupon solutions, even up to 50% of the overhead over the network. Moreover, we solve other issues related to the viability of these schemes, for example, using a standardized way to distribute non-standard cryptographic key material. Therefore, the viability of a multicoupon solution should be validated considering all involved factors, such as internal processing, networking, message encoding and current technology support.

     

盲人手机输入方案的交互设计和评估

以交互设计模式对盲人手机界面的输入方式进行了研究,按照交互设计流程对盲人手机进行需求分析,提出了盲人手机输入方式的两个候选方案,并利用非语音声音表示数据的听觉显示技术在Windows图形界面上建立了原型.利用原型以迭代方式对方案进行了两轮评估和改进,展望了候选方案的研究前景.     

一种改进的2pc协议及其性能

提出了一种改进的事务提交协议pg2pc(谨慎成组两阶段提交协议),支持成组全局事务处理,与传统的2pc相比降低了事务提交过程的消息量,提高了事务处理的吞吐量和可靠性,同时对节点故障问题提出解决方案。     

Deriving protocols for services supporting mobile users

A Prolog tool for automated derivation of protocol specifications from service specifications is described. The server for which the protocol is derived may consist of any finite number of protocol entities co-operating over reliable unbounded first-in-first-out channels. Its service is expected to consist of service primitives that read or write unstructured global virtual variables, implicitly receive or compute their current values or delete their local copies. In addition, service primitives may access distributed virtual queues, to which they append elements with desired priority or consume their head elements. Service users are allowed to dynamically select the service-access point through which they interact with the distributed server. The adopted specification language has been inspired by LOTOS.     

Termination detection protocols for mobile distributed systems

This paper studies a fundamental problem, the termination detection problem, in distributed systems. Under a wireless network environment, we show how to handle the host mobility and disconnection problems. In particular, when some distributed processes are temporarily disconnected, we show how to capture a weakly terminated state where silence has been reached only by those currently connected processes. A user may desire to know such a state to tell whether the mobile distributed system is still running or is silent because some processes are disconnected. Our protocol tries to exploit the network hierarchy by combining two existing protocols together. It employs the weight-throwing scheme on the wired network side, and the diffusion-based scheme on each wireless cell. Such a hybrid protocol can better pave the gaps of computation and communication capability between static and mobile hosts, thus more scalable to larger distributed systems. Analysis and simulation results are also presented     

Efficient validation of mobile transactions in wireless environments

In broadcast environments, the limited bandwidth of the upstream communication channel from the mobile clients to the server bars the application of conventional concurrency control protocols. In this paper, we propose a new variant of the optimistic concurrency control (OCC) protocol that is suitable for broadcast environments. At the server, forward validation of a transaction is done against currently running transactions, including mobile transactions and server transactions. At the mobile clients, partial backward validation of a transaction is done against committed transactions at the beginning of every broadcast cycle. Upon completion of execution, read-only mobile transactions can be validated and committed locally and update mobile transactions are sent to the server for final validation. These update transactions have a better chance of commitment because they have gone through the partial backward validation. In addition to the nice properties of conventional OCC protocols, this protocol provides autonomy between the mobile clients and the server with minimum upstream communication, which is a desirable feature to the scalability of applications running in broadcast environments. This protocol is able to process both update transactions and read-only transactions at the mobile clients at low space and processing overheads.     

Design and performance evaluation of throughput-aware rate adaptation protocols for IEEE 802.11 wireless networks

Experimental studies have shown that traditional rate adaptation schemes for 802.11 wireless networks suffer from significant throughput degradation in highly congested networks. To address this problem, this paper makes the following two main contributions. First, we design a method to accurately estimate the per-packet transmission times, and we use these measurements to provide a broad classification of the network state and to identify network congestion. Second, we design a new Throughput-Aware Rate Adaptation (TARA) scheme, which uses the congestion estimates to mitigate the negative impact of link-layer collisions on the operations of rate adaptation, without requiring changes in the 802.11 MAC specification. Another key feature of our solution is to minimize probing overheads and limit unnecessary rate decreases by predicting the throughput gain that could be brought about by a change in the transmission rate. Through experiments conducted across a variety of network scenarios and traffic patterns, we show that TARA achieves significantly higher throughput than the other rate adaptation algorithms implemented in the legacy Madwifi driver.     

A reliable multicast protocol for distributed mobile systems:design and evaluation

Reliable multicast is a powerful communication primitive for structuring distributed programs in which multiple processes must closely cooperate together. We propose a protocol for supporting reliable multicast in a distributed system that includes mobile hosts and evaluate the performance of our proposal through simulation We consider a scenario in which mobile hosts communicate with a wired infrastructure by means of wireless technology. Our proposal provides several novel features. The sender of each multicast may select among three increasingly strong delivery ordering guarantees: FIFO, causal, total. Movements do not trigger the transmission of any message in the wired network as no notion of hand-off is used. The set of senders and receivers (group) may be dynamic. The size of data structures at mobile hosts, the size of message headers, and the number of messages in the wired network for each multicast are all independent of the number of group members. The wireless network is assumed to provide only incomplete spatial coverage and message losses could occur even within cells. Movements are not negotiated and a mobile host that leaves a cell may enter any other cell, perhaps after a potentially long disconnection. The simulation results show that the proposed protocol has good performance and good scalability properties