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题名:
互联网中的服务质量保证
作者: 石志强
答辩日期: 2001
专业: 计算机应用技术
授予单位: 中国科学院软件研究所
授予地点: 中国科学院软件研究所
学位: 博士
关键词: 服务质量 ; 拥塞控制 ; 集成服务 ; 区分服务 ; 分组交换 ; 实时多媒体 ; 随机早期 ; 检测 ; 基于启动势的公平队列调度 ; 基于启动势的延迟抖动公平队列调度 ; 资源预留协议 ; 发送者预留协议 ; 带宽分配 ; 资源预留汇聚 ; 组播
摘要: TCP的拥塞控制极大地促进了互联网的发展,它成功地满足了FTP, WWW,Email等以数据传输为主的应用对服务质量的需要,随着网络设备处理能力的增强,公平队列、随机早期检测等队列调度算法进一步提供了网络资源共享的公平性和高效性。目前,IP电话、远程教育等实时多媒体应用已成为未来互联网发展的方向,为实时应用提供服务质量保证的相关网络技术也逐步走向成熟。本文研究了如下领域:1 传统业务的拥塞控制和队列调度。本文介绍了基于端系统的 TCP 拥塞控制的发展历程,延迟确认和 UDP业务等对 TCP的影响,公平队列、随机公平队列等多队列调度算法,并对 RED 单队列调度算法作了详细的分析,阐述了 RED 的参数设置原则及其复杂性。本文提出了一种估计和设置 RED 参数的方法,使之勇动态适应当前网络流量的要求,研究了在严重拥塞环境下,RED实际丢包率变化引起的网络流量震荡问题,给出了相应的改进算法,并通过动态仿真,证明该改进算法提高了RED的适应性,降低了网络时延,减小了时延抖动。2 多媒体业务的队列调度。实时多体业务需要严格的服务质量保证,如带宽、时延和时延抖动等,一旦服务质量低于应用的需要,业务将难以继续。GPS 调度算法可以提供零服务等特和绝对的公平性,但 GPS 只是一种理想的队列调度算法,在实际网络中是无法实现的。在分组交换网络中,GPS的离散形式WPQ也具有很好的延迟和较好的公平特性,但由于其计算复杂度为 O(N),难以被广泛使用;目前的最新研究成果是 SPFQ 算法,它的计算复杂度为 O(log_2 N),并且可以提供与 WFQ相似的服务质量。本文分析了 SPFQ的公平和延迟抖动特性,发现它们的会话间依赖性较强,并提出了它的一种改进算法 SPJFQ 它在 SPFQ 算法的基础上,添加了允许控制,即只有数据包的启动势大等于当前系统势,该数据包才能发送,SPJFQ 的计算复杂度仍为 O(log_2 N),却具有更好的公平和延迟抖动特性,它们都不依赖于其它会话资源预留特性。3 集成服务的实现与改进。集成服务 IntServ[RFC1633]是由 IETF 的IntServ 工作组于1994年提出的实现规范,其目的在于建立一个能同时为实时和非实时业务提供服务的分组交换网络。集成服务采用资源预留协议 (RSVP) 作为其接入控制手段,通常采用 WFQ 等队列调度机制保证数据传输的带宽和时延。我们采用基于 CBQ 的队列调度算法,在Linux下实现了资源预留协议,它可以提供负载受控的服务和保证质量的服务,通过测试我们发现在会话建立过程中,底层链路操作占用的处理器开销最大。本文还进一步比较了YESSIR和RSVP的差异,分析了它们各自的合理因素,并提出了RSVP的改进方案。它不仅保留了接收者预留、软状态和支持组播等特性,还具有更小的刷新处理开销,并避免了资源建立过程中频繁的链路操作。4 区分服务的研究与改进。区分服务是IETF定义的另一种 QoS机制,它不象集成服务那亲为每个数据流预留资源,而是把数据流聚类到不同的服务等级,对每个等级的数据包提供相应的服务。核心网络由于会话量大,链路速度快,集成服务的队列调度和接入控制都难以满足实际的需要。区分服务是公认的、适宜于核心网络的服务模型,但目前它只定义了一些数据传输的处理行为,未规范控制的策略。我们比较了带宽分配 (BB) 和资源预留汇聚两种接入控制方案的差异,带宽分配方案可能会有较小的空间复杂度,而资源预留汇聚却更能适应组播通信的需要。资源预留汇聚虽然可以解决组播通信环境的接入控制问题,但由于区分服务流量控制能力较弱,仍难以满足组播通信环境下高优先级数据传输的需要。为此,我们提出了基于路由表的流量控制策略,它有效地解决了组播环境下,服务质量保证的难题。
英文摘要: The congestion control of TCP greatly accelerated the development of the Internet. It successfully provides satisfactory quality of service to the data applications such as FTP, WWW and Email. With the building up of the processing power of the network devices, queuing algorithms such as fair queuing and random early detection further provides fairness and high efficiency for network resource sharing. At present real-time multimedia applications such as IP telephoning and distant education has become the future direction of the Internet, and the relevant network technologies for providing quality of service guarantees to real-time applications are gradually maturing. This thesis mainly focuses on the following areas: 1. Congestion control and queuing for traditional traffic. This thesis gives an introduction of the development of the end system's TCP congestion control, the impact of delayed ACK and UDP traffic on TCP, and multi-queue scheduling algorithms such as fair queuing and random fair queuing. After this, we give a detailed analysis of the RED algorithm. We elaborate on the parameter setting principles and the complexity of the algorithm. Based on our analysis, this thesis presents a method for estimating and setting the RED parameters, which enables the algorithm to adapt to the current network traffic. We have also investigated the traffic oscillation problem caused by actual RED packet loss rate under heavily congested network environment. We present an improved algorithm that addresses this problem. Through dynamic simulation, we have also proven that our improvement can enhance RED's adaptability as well as reduce the network delay and delay jitter. 2. Queuing for multimedia traffic. Real-time multimedia traffic requires strict quality of service guarantees such as bandwidth, delay and delay jitter. Once the QoS is lower than needed, the service will be unable to continue. The GPS scheduling algorithm can provide zero latency and absolute fairness. But GPS is only an ideal algorithm in theory and can not be implemented in practice. In packet switching networks, WFQ, the discrete form of GPS, also has good delay and fairness, but its computing complexity is O(N), which makes it unable to be widely deployed. The most up-to-date research result is the SPFQ algorithm. Its computing complexity is O(log_2 N) and can provide quality of service similar to WFQ. This thesis analyzes the fairness and delay jitter characteristics of SPFQ. We find that there is a strong inter-session dependence. We present a further improved SPJFQ algorithm, which enhances the SPFQ algorithm by adding eligibility control A packet is sent only when its starting potential is larger than the system potential. The computing complexity of SPJFQ is also O(log_2 N), but it has better fairness and delay jitter characteristics, which are independent of the resource reservation of other sessions. 3. Implementation and improvement of the Integrated Services. Integrated Services is a specification proposed by the IETF IntServ working group in 1994. The aim is to construct a packet switching network which can provide service to both real-time and non real-time traffic. IntServ adopts the resource reservation protocol (RSVP) as the admission control, and usually employs scheduling mechanisms such as WFQ to provide bandwidth and delay guarantees of data transmission. We have implemented the RSVP protocol under Linux with CBQ as the scheduling algorithm. Our implementation can provide controlled load service and guaranteed service. Through much testing, we find that the link layer operations constitute the largest overhead during the session set up process. Research and improvement of Differentiated Services. The Differentiated Services is another QoS mechanism proposed by the IETF. Unlike IntServ, which reserves resource for each data flow, DiffServ aggregates data flows to different service levels and provides service to data packets according to their service level. Because of the large volumes of sessions and the extremely fast link in core networks, the scheduling and admission control of IntServ are impractical under this environment. DiffServ is a service model that is widely accepted to be suitable for core networks. Currently DiffServ has only defined the behavior of data transmission. There has been no specification for admission control. We made a comparison of the two admission control solutions, namely the Bandwidth Broker (BB) and the resource reservation. We find that BB may have smaller space complexity, while resource reservation can better serve multicast communication. 4. Research and improvement of Differentiated Services. The Differentiated Services is another QoS mechanism proposed by the IETF. Unlike IntServ, which reserves resource for each data flow, DiffServ aggregates data flows to different service levels and provides service to data packets according to their service level. Because of the large volumes of sessions and the extremely fast link in core networks, the scheduling and admission control of IntServ are impractical under this environment. DiffServ is a service model that is widely accepted to be suitable for core networks. Currently DiffServ has only defined the behavior of data transmission. There has been no specification for admission control. We made a comparison of the two admission control solutions, namely the Bandwidth Broker (BB) and the resource reservation. We find that BB may have smaller space complexity, while resource reservation can better serve multicast communication. Although aggregation of resource reservation can solve the problem of admission control for multicast, it can not satisfy the needs of high priority data transmission because of the weak traffic control capability in DiffServ networks. We have presented a traffic control mechanism based on the routing table, which efficiently solves the QoS guarantee problem in the multicast environment.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.iscas.ac.cn/handle/311060/6528
Appears in Collections:中科院软件所

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Recommended Citation:
石志强. 互联网中的服务质量保证[D]. 中国科学院软件研究所. 中国科学院软件研究所. 2001-01-01.
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