一种有效的实时选播流连接建立算法

选播服务被定义为一组复制服务器所提供的相同或类似的服务。使用选播服务能大大简化某些应用，例如查找合适的服务器来获得QoS服务、实现网络负载平衡和客错性服务等。为选播流建立连接提出了一种有效的分布式算法。对于有时延约束和最小带宽要求的一个选播流，我们的算法能有效地并行查找多个目标以建立连接、并确保满足选播请求的最好路径得到选择。     

城域网上CDN技术的应用

CDN是一个建立并覆盖在互联网,并由分布在不同区域的节点服务器群组成的虚拟网络。CDN采用缓存、复制、负载均衡和客户请求重定向等技术,将信息资源推向网络边缘,使得客户可以从"最近最好"的服务器快速访问到所需的内容,从而提高了终端用户的访问速度。文中简要介绍CDN的概念和技术,包括CDN基本工作原理,内容路由的设计原则和机制比较,节点内容引擎比较。通过CDN的技术方案,为宽带的发展提供一些技术参考。     

调度算法在节点延时PMF正态化条件下的延时分析

若假设网络带宽足够大,可以将网络传输的QoS保证表示为以网络延时保证为前提,以丢包率为判决条件.本文分析网络现有调度算法对网络业务延时的影响,选择节点状态延时的概率函数（PMF）表示网络节点状态的统计信息.并利用其PMF的正态形式作为网络节点状态信息,分析调度算法的延时性能.通过仿真验证,当节点延时的PMF分布近似正态,即如果网络业务经过整形后以近似正态延时概率到达网络节点,业务在节点经过调度算法输出时其PMF仍近似保持正态分布形式.仿真分析了这种情况下各调度算法的延时表现,得到EDF算法在这种情况下延时随节点级数增加的特性最好.     

一种光盘服务器两级Cache算法的研究与实现

随着网络上光盘资源的增长，光盘服务器成为光盘网络共享的重要技术,针对传统光盘服务器的缺点，实现了一个新的高性能光盘服务器CDS（CDServer）,CDS系统采用两级Cache（客户端Cache加服务器Cache）的技术来提高系统的性能．客户端Cache根据光盘顺序访问特点，采用慢速增长快速下降的预取算法设计，即提高了系统的性能，也保证预取不命中时的响应时间，服务器Cache采用Hash算法同平衡二叉树相结合的两级组织结构，实现了Cache的快速查找．在详细介绍了CDS系统的两级Cache算法的同时，进行了相应的试验测试和性能分析．     

Internet上一种有效的QOS选播路由算法

为了提高网络服务的范围和服务质量 ,在 Internet经常采用一组复制服务器的方法。选播服务是一种新的网络服务模型 ,可以改善网络负载分布和简化某些网络应用。本文介绍了一个简单的基于 Internet选播服务模型 ,该模型对目前使用的网络路由算法和协议的影响不大 ,并且提出了服务于该模型的 Qo S路由算法 ,该算法采用随机策略来平衡网络负载和改善网络性能。在不同网络拓扑和不同网络参数下的模拟结果表明该算法具有良好的服务性能 ,并能有效平衡网络负载     

抖动环境下文件复制节点选择算法

文件复制技术是把负载分散在复制节点上,避免服务器超载,减少服务器响应延迟和查询路径的长度,以达到较高查询率的有效方法。因此文件复制节点的选择就成为了首要考虑的问题。P2P文件共享系统因为节点的加入和离开,甚至节点失效造成拓扑结构会不断地变换,选择一个文件处理性能好,抖动性弱的节点作为复制节点就显得非常重要。提出一种新的文件复制节点选择算法NDC(New Dynamic and Capacity Algorithm),通过在复制节点的选取上考虑抖动性因素,减少节点离开或者失效造成较长的通信延迟和用户体验的降级,提高文件击中率。由于网络的热点文件不断变化,会出现不同倾斜查询的情况,对NPC处理倾斜性查询文件的能力进行了分析。通过在P2Psim上进行的大量实验表明,NPC算法选出的节点处理文件性能好,抖动性弱,可以提供更精确的文件复制节点。     

基于VxWorks的NTP服务器的设计与实现

针对民航报文处理系统对时间同步的要求，采用VxWorks嵌入式操作系统，设计并实现了多工作模式的网络授时协议（NTP）服务器，建立了可靠的处理机制；并在网络授时协议的基础上，改进了广播模式实现算法和模式转换算法。最后通过测试对服务器的性能进行了评估。测试结果表明，基于VxWorks操作系统的网络授时协议服务器具有实时性好，稳定性高等优点。     

基于反馈控制理论的流媒体服务器资源管理模型

如何合理地分配系统资源，使流媒体服务器能够在满足客户基本QoS的前提下，支持尽可能多的并发服务，是流媒体研究领域的重要组成部分．提出了基于反馈控制理论的流媒体服务器资源调度模型及核心调度算法．该模型通过周期地采集系统的工作状态，并以此为根据调整任务在下一个采样周期内的运行属性，达到在满足任务的QoS的前提下，提高系统的并发程度的目的．详细分析了模型结构和核心算法的实现机制，并利用模拟平台对该算法进行了性能分析．     

高可用冗余TCP协议堆栈

提出了一个冗余TCP堆栈(RTS)方案以提高TCP网络连接的可靠性和计算机集群提供服务的可用性．RTS允许多个服务器同时在一个TCP连接里工作，极大的改善了TCP连接提供的服务的可信赖性．在RTS中还设计了两个算法(SWMU和SWFR)来解决在多个TCP协议堆栈中的同步问题．实验表明，使用SWFR和BRC算法可以将网络性能的损耗减到最少，并且配备两台服务器的集群所发挥的性能可充分满足在实际使用中所需的高效传送．     

多服务器模型下的服务器选择算法及仿真

在分布式视频点播系统中,由于不同视频服务器到代理服务器之间的网络QoS状况可能存在较大的差异,因此多媒体代理服务器必须引入有效的服务器选择算法。文章主要讨论所提出的在多服务器模型下基于增益模型的服务器选择算法;通过仿真实验测试其性能、与其他算法进行比较;并分析此算法对环境参数变化的适应性。     

基于博弈论的多服务器多用户视频分析任务卸载算法

在过去的近10年中,人工智能相关的服务和应用大规模出现,它们要求高算力、高带宽和低时延.边缘计算目前被认为是这些应用最适合的计算模式,尤其是视频分析相关应用.研究多服务器多用户异构视频分析任务卸载问题,其中用户选择合适的边缘服务器,并将他们的原始视频数据上传至服务器进行视频分析.为了有效处理众多用户对有限网络资源的竞争和共享,并且能够获得稳定的网络资源分配局面,即每个用户不会单方面地改变自己的任务卸载决策,该多服务器多用户异构视频分析任务卸载问题被建模为一个多玩家的博弈问题.基于最小化整体时延的优化目标,先后研究非分布式视频分析场景和分布式视频分析场景两种情形,分别提出基于博弈论的潜在最优服务器选择算法和视频单元分配算法.通过严格的数学证明,两种情形下提出的算法均可以达到纳什均衡,同时保证较低的整体时延.最后,基于真实数据集的大量实验表明,所提方法比其他现有算法降低了平均26.3%的整体时延.     

End-to-end latency of a fault-tolerant CORBA infrastructure

This paper presents an evaluation of the end-to-end latency of a fault-tolerant CORBA infrastructure that we have implemented. The fault-tolerant infrastructure replicates the server applications using active, passive and semi-active replication, and maintains strong replica consistency of the server replicas. By analyses and by measurements of the running fault-tolerant infrastructure, we characterize the end-to-end latency under fault-free conditions. The main determining factor of the run-time performance of the fault-tolerant infrastructure is the Totem group communication protocol, which contributes to the end-to-end latency primarily in two ways: the delay in sending messages and the processing cost of the rotating token.To reduce the delay in sending messages for passive and semi-active replication, the position of the primary server replica on the Totem ring, the token rotation time, the processing time at the client, and the processing time at the server must be considered. For active replication, the presence of duplicate messages adversely affects the performance. However, if an effective sending-side duplicate suppression mechanism is implemented, active replication is more advantageous than both passive and semi-active replication because of the automatic selection of the most favorable position of the server replica that sends the first non-duplicate reply.     

Video Packet Selection and Scheduling for Multipath Streaming

This paper addresses the problem of choosing the best streaming policy for distortion optimal multipath video delivery, under network bandwidth and playback delay constraints. The streaming policy consists in a joint selection of the network path and of the video packets to be transmitted, along with their sending time. A simple streaming model is introduced, which takes into account the video packet importance, and the dependencies between packets. A careful timing analysis allows to compute the quality perceived by the receiver for a constrained playback delay, as a function of the streaming policy. We derive an optimization problem based on a video abstraction model, under the assumption that the server knows, or can predict accurately the state of the network. A detailed analysis of constrained multipath streaming systems provides helpful insights to design an efficient branch and bound algorithm that finds the optimal streaming strategy. This solution allows to bound the performance of any scheduling strategy, but the complexity of the algorithm becomes rapidly intractable. We therefore propose a fast heuristic-based algorithm, built on load-balancing principles. It allows to reach close to optimal performance with a polynomial time complexity. The algorithm is then adapted to live streaming scenarios, where the server has only a partial knowledge of the packet stream, and the channel bandwidth. Extensive simulations show that the proposed algorithm only induces a negligible distortion penalty compared to the optimal strategy, even when the optimization horizon is limited, or the rate estimation is not perfect. Simulation results also demonstrate that the proposed scheduling solution performs better than common scheduling algorithms, and therefore represents a very efficient low-complexity multipath streaming algorithm, for both stored and live video services     

一种更安全的基于智能卡的多服务器身份认证方案研究

随着网络和通讯技术的发展,多服务器的身份认证问题成为近几年研究的重点。针对Chuang等人提出的基于生物特征的多服务器密钥认证方案存在的安全性不足等问题,Wang等人进行了改进,但是仍未解决不同服务器密钥相同造成的恶意攻击和伪造攻击等问题。因此,本文提出了一种更安全的基于智能卡的多服务器身份认证方案。该方案在服务器注册阶段,将高熵秘密数与服务器的ID进行计算再重新分配给服务器,使得不同的服务器具有不同的密钥。最后通过五个方案功能和性能比较分析,得出改进方案比前三种方案时间上分别缩短了16.67%、20%、28.57%,且能有效阻止恶意服务器攻击、伪造攻击、重放攻击和中间人攻击等多种攻击,安全性得到了提高,满足实际网络的高可靠性需求。     

Analysis of a class of distributed queues with applications

Recently we have developed a class of media access control algorithms for different types of Local Area Networks. A common feature of these LAN algorithms is that they represent various methods by which the processors in the LAN can simulate the availability of centralized packet transport facility, but whose service incorporates a particular type of changeover time known as ‘moving server’ overhead. First we describe the operation of moving server systems in general, for both First-Come-First-Served and Head-of-the-Line orders of service, together with an approach for their delay analysis where we transform the moving server system into a conventional queueing system having proportional waiting times. Then we describe how the various LAN algorithms may be obtained from the ideal moving server system, and how a major component of their performance characteristics is determined by the performance characteristics of that ideal system. Finally, we evaluate the compatibility of such LAN algorithms with separable queueing network models of distributed systems by computing the interdeparture time distribution for M/M/1 in the presence of moving server overhead. Although it is not exponential except in the limits of low server utilization or low overhead, the interdeparture time distribution is a weighted sum of exponential terms with a coefficient of variation not much smaller than unity. Thus, we conjecture that a service centre with moving server overhead could be used to represent one of these LAN algorithms in a product form queueing network model of a distributed system without introducing significant approximation errors.     

Periodic broadcast with dynamic server selection

Service replication is an effective way to address resource requirements and resource availability problem. Dynamic service selection enables clients to choose a server offering the best performance. Proper server selection is especially important for video streaming over the Internet due to its high bandwidth requirements. However, given the length of a typical video transmission, the server priorly selected may no longer be an optimal one for the duration of the entire transmission. More importantly, a server may fail during the transmission of a video. In this paper we examine the possibility of switching to another server during an on-going transmission for Periodic Broadcast schemes. Due to the timing requirements typical for Periodic Broadcast the server switch may cause playback disruptions. We analyze the magnitude of the problem and propose an easy to implement solution. We define the criteria, additional to the bandwidth availability for example, according to which a new server should be selected. The client is also required to delay its playback by the amount of time bounded by the server transmission offset. In addition, we propose an alternative method to ensure uninterrupted playback that relies on proxy caching. Simulation results show that our approach can significantly reduce the likelihood of playback disruptions.

多服务副本下的一种分布式负载均衡路径算法

目前的服务器负载均衡算法多只针对选择一台服务器提供所有服务,不能满足服务组合需要选择一组服务副本的情况。针对整个集群系统拥有多台服务器,每台服务器具有多个服务副本的服务查找与组合的路径选择和负载均衡问题,提出了一种基于节点负载变化的分布式负载均衡路径算法LCNA（Load Changes of Notes-considered Algorithm）。LCNA算法以基于DHT的路由算法来建立集群系统的服务列表和路由转发列表,以服务器节点的动态负载变化作为测度进行服务节点的选择。实验结果表明,LCNA算法能有效提高多服务副本环境下应用的整体性能,具有良好的负载均衡效果。     

海量数据分布式处理的研究与实现

介绍了涉及海量数据的大型软件的分布式解决方案。将繁重任务分解到Internet／Intranet上的多台机器提高性能是分布式系统的出发点,借此思想处理海量数据、实现数据的分开配置。对DCOM组件的实现原理、组件的划分、远程组件服务器的配置以及采用MMF处理超大型文件进行了详细阐述,而且该方案极易应用到实际开发中,对同类问题极具参考性。     

Online Bicriteria Load Balancing Using Object Reallocation

We study the bicriteria load balancing problem on two independent parameters under the allowance of object reallocation. The scenario is a system of $M$ distributed file servers located in a cluster, and we propose three online approximate algorithms for balancing their loads and required storage spaces during document placement. The first algorithm is for heterogeneous servers. Each server has its individual tradeoff of load and storage space under the same rule of selection. The other two algorithms are for homogeneous servers. The second algorithm combines the idea of the first one and the best existing solution for homogeneous servers. Using document reallocation, we obtain a smooth tradeoff curve of the upper bounds of load and storage space. The last one bounds the load and storage space of each server by less than three times of their trivial lower bounds, respectively; and more importantly, for each server, the value of at least one parameter is far from its worst case. The time complexities of these three algorithms are $O(log M)$ plus the cost of document reallocation.     

Assignment of Movies to Heterogeneous Video Servers

A video-on-demand (VOD) system provides an electronic video rental service to geographically distributed users. It can adopt multiple servers to serve many users concurrently. As a VOD system is being used and evolved, its servers probably become heterogeneous. For example, if a new server is added to expand the VOD system or replace a failed server, the new server may be faster with a larger storage size. This paper investigates how to assign movies to heterogeneous servers in order to minimize the blocking probability. It is proven that this assignment problem is NP-hard, and a lower bound is derived on the minimal blocking probability. The following approach is proposed for assignment: 1) problem relaxation—a relaxed assignment problem is formulated and solved to determine the ideal load that each server should handle, and 2) goal programming—an assignment and reassignment are performed iteratively while fulfilling all the constraints so that the load handled by each server is close to the ideal one. This approach is generic and applicable to many assignment problems. This approach is adopted to design two specific algorithms for movie assignment with and without replication. It is demonstrated that these algorithms can find optimal or close-to-optimal assignments.