【作者】 彭昭；

【导师】 杨宗凯；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2008， 博士

【摘要】 近年来网络流媒体应用日趋普及,媒体分发技术的边下载边播放特性对网络带宽和服务器资源提出了较高的要求,随着日益增加的用户和视频数据,如何在因特网上提供大规模流媒体服务已成为流媒体研究领域的核心问题。传统C/S模式服务器很容易成为系统瓶颈。IP组播采用多个Peer节点共享媒体流的方式减轻服务器和网络负载,但由于部署原因很难在因特网上广泛实施。CDN采用代理缓存节点的方式将服务和内容推向网络“边缘”,但部署昂贵且存在瓶颈。P2P模式的引入为用户提供了高度灵活的交互环境,允许协作节点间共享计算、带宽和存储等资源,彻底改变了视频点播的服务模式,显著地减少了服务器负载,提高了网络资源利用率,但参与节点的动态性制约了P2P系统的扩展与大规模应用。在现有网络条件下,如何提高大规模P2P点播数据分发质量和效率,已成为P2P流媒体研究领域的重要课题之一。针对P2P视频点播系统的数据实时性、节点互动性、资源和请求异构性以及VCR等动态行为,本文研究非结构化P2P点播数据分发技术,主要包括以下三个问题:(1)非结构化的P2P点播服务体系,解决点播系统中数据资源和剩余带宽利用率低,延时难以控制的问题;(2)高码率视频传输调度方案,用以满足高端用户的高质量视频服务需求;(3)大规模P2P点播分发中的用户激励机制,充分考虑点播系统中视频数据实时性、上下游节点请求异构及资源不对等特性。具体的研究内容如下:非结构化的P2P点播服务体系。基于扩展性考虑,提出非结构化拓扑的P2P点播系统AVoD,以数据驱动构成覆盖网络。采用异步传输策略提高可用节点资源利用率,减少离散资源允布对P2P点播系统性能的影响。鉴于异步传输非实时性特点与视频实时性的矛盾,在满足“多源”节点数据传输以及一定下载速率的条件下,引入优先级策略建立一种基于服务质量与效率平衡的节点选择算法,以期解决由此带来的视频播放连续度问题。基于服务器负载理论分析推导出服务器压力与缓存大小以及高低优先级分配比例的关系,进一步验证基于非结构化P2P点播异步传输系统的扩展性。仿真实验表明系统在随机、突发等动态应用场景中具备较好的应用性能。高码率P2P点播分发中的补偿调度。针对P2P点播系统的高码率视频数据分发问题,该研究点主要侧重于高码率数据传输延迟研究,并基于节点定位和数据传输时延提出解决思路。通过预估节点缓存区间,在保证一定节点定位精确度的前提下,减少定位时可能产生的更新操作,进而更进一步减少由此产生的定位延迟。引入补偿服务器机制,通过补偿服务器对初始视频和部分空缺视频进行补偿,解决大规模网络条件下可能产生的链路阻塞和资源“饥饿”问题。并在此基础上,提出补偿流和正常流的协作机制与流程,以及补偿服务器的缓存策略。仿真实验表明在合理的服务器压力前提下,引入补偿策略能够很好的改善系统播放性能。大规模P2P点播分发中的激励机制。基于点播系统的视频数据实时性、上下游节点请求异构及资源不对等特性的考虑,针对非结构化P2P点播系统的节点自私行为,提出一种激励机制,通过差异服务抑制自私节点并促使更多节点共享资源,给出基于贡献度和请求队列长度的节点选择概率算法,并在此基础上提出源节点和请求节点的激励数据调度策略。此外,分析节点可能的“漂白行为”,通过区分对待新加入节点的贡献度统计,抑制引入激励可能衍生的节点自私现象。仿真实验表明激励机制的引入能够较好的改善系统播放性能,并抑制自私节点获取更多的“免费”资源。本文得到了“下一代互联网(CNGI)示范项目(基于P2P的数字媒体业务研发及应用实验)”项目支持,研究结果可用于点播应用性能和网络性能的统计分析,并为P2P大规模点播应用和示范部署提供参考依据。更多还原

【Abstract】 Large-scale media distribution technology of VoD(Video-on-Demand) has broad application scenarios.There are some requirements of high bandwidth and long duration for multimedia streams due to diversity requests from users and real-time data transmission from video sequences.So how to provide large-scale media service in a scalable way has been a core issue in the multimedia research area with the growing number of users and videos.In the traditional C/S(Client/Service) system,the centralized server can easily become the bottleneck.IP Multicast reduces the workload of source server and transmission network by having,multiple peers sharing the media streams, however,its deployment on the interact is still limited due to several fundamental concerns.CDN(Content Distribution Network) pushes the service and content to the edge of the internet by utilizing agent cache nodes,but there are also some bottlenecks because of the expensive deployments.P2P mode can provide users with highly flexible interactive environments.Since P2P introduces the sharing of bandwidth,storage and other resources among peers,it has completely changed the VoD service mode,significantly reduced the server load,and greatly improved the utilization of network resources.But the scalability and large-scale application of P2P VoD system remains to be restricted by the dynamic of peers.How to improve the quality and efficiency of data transfer for large-scale P2P VoD applications has become a "hot spot" in the research community of P2P VoD system. Based on the characteristics of real-time data transfer,peer interaction,diversity requests of resources,and VCR operations,this thesis investigates the distribution technology of unstructured P2P VoD,including three aspects:(1) Unstructured-based P2P VoD service architecture,which is used to address the low utilization of data source and remaining bandwidth,as well as uncontrollable playback delay.(2) High bit-rate video transfer scheduling scheme,which is used to meet the requirements on high quality video service for high-level users.(3) Incentive in large-scale P2P VoD distribution,which takes full account of real-time data,heterogeneous request and unequal content from upstream and downstream.Unstructured-based P2P VoD service architecture.The dissertation proposes a P2P VoD system with unstructured topology for the consideration of scalability,and the overlay topology is a data-driven network.The scheme of asynchronous data transfer specializes in effective mining of available video data source at the cost of playback continuity.And it could also reduce the impacts on playback performances from discrete distribution of resources.In view of the contradictions between the characteristics of non-real-time in asynchronous transfer and the real-time in video playback,a novel priority strategy is introduced to establish a compromising selecting algorithm for peers, and to solve the continuity issues for VoD system on condition of meeting normal data transfer and acceptable download rate for multiple peers.The theoretical analysis of server workload points out the relationship between server stress and the peer buffer size as well as the ratio of high priority.The simulation results further show the benefits in terms of continuity,hunch delay and server stress in random and sudden scenarios.Compensation-assisted scheduling in high-quality P2P VoD distribution.To solve the data scheduling issues for high bit-rate streams,we take the analysis and summary from two perspectives of heterogeneous resources and network restrictions in large-scale applications,and propose solutions based on the delay of peer locating and data transfer. An estimate algorithm for peer buffer size is introduced from the research on the update delay in centralized locating strategy.Under accurate locating conditions,the algorithm can lower the update frequency of peers,and then reduce the workload of directory server and update delay of peer locating.Further more,the compensation-assisted strategy,as well as the collaborate process of compensation stream and cache strategy of compensation server,are given to solve resource "hunger" caused by the link jam and asymmetry.The simulation results show that the introduction of compensation strategy can improve the system playback performance on the premise that the server workload is acceptable.Incentive in large-scale P2P VoD distribution.To solve the free-riding issues among peers and stimulate more peers to contribute their bandwidth and storage,an incentive based on the service differentiation by the contribution of peers is proposed for unstructured P2P VoD systems.In this incentive,the contribution of a peer was measured by peer contribution considering both the requirement of real-time video playback and the resource inequality of peers.Meanwhile,a service differentiation scheme was introduced in data transfer scheduling based on the contribution measure of requesting peer and the request queue length of supplying peers.Also,an identification mechanism was presented to avoid possible whitewashing behaviors of peers.Simulation results show that the proposed incentive is effective to constrain the selfish behaviors of peers,and improve the performances of demanded video playback.The works in this thesis have been supported by the China Next Generation Internet "P2P-based digital media business research and application experiment".It is expected that the research results can be applied to the statistical analysis on application and network performance for P2P VoD,and can provide a reference for deployments of large-scale P2P VoD application.更多还原