博弈论框架下P2P实时流的合作模型研究

针对P2P实时流环境中出现的“搭便车”和“公共悲剧”问题,提出一种博弈论框架下的激励合作模型。分析该模型达到Nash均衡和Pareto最优状态下对应的比例公平策略优化。并考虑存在欺骗行为的情况,研究对应的节点行为策略。理论分析表明,该模型能够刺激节点合作,并且对节点的欺骗行为具有抑制作用。     

一种基于节点全局信任值的BitTorrent系统区分服务机制

BitTorrent系统中节点的搭便车行为严重影响系统的健壮性和稳定性.针对BitTorrent系统中搭便车行为抑制机制存在的不足,提出一种基于节点全局信任值的区分服务机制.在该机制中,Tracker服务器依据节点反馈的本地信任值计算节点全局信任值,依据全局信任值识别贡献节点与搭便车节点,对两类节点采取区分服务策略:为贡献节点与搭便车节点分别返回其同类对等节点,使搭便车节点无法与贡献节点建立连接进而获取文件资源;在贡献节点中又进一步区分高贡献节点和普通贡献节点,引入返回比例因子为其提供差异服务,使高贡献节点获得更快的下载速度.实验表明,该机制能有效隔离并惩罚搭便车节点,奖励高贡献节点.     

基于BT协议的网络激励机制研究

由于p2p网络节点的匿名性和贡献资源的自愿性,绝大多数节点不愿共享自己的资源,从而导致大量搭便车现象的出现。在使用BitTorrent下载的过程中,为了能得到对方更快的下载速率,各个节点也能够为相应节点提供较快的下载速率,但在下载完成以后,大部分节点就不再为其他节点提供下载服务。在分析BitTorrent协议的基础上,该文提出了一种差异服务的激励机制,该激励机制能够激励节点持续不断地为其他节点提供下载服务,即使是在该节点下载完成以后也是如此。     

一种缩短下载时间优先的自适应BitTorrent激励协议

BitTorrent激励机制的目标是保证节点上传和下载之间的公平性,但相比公平性而言,实际应用中的节点更优先考虑的是文件下载时间,据此文中提出了一种缩短文件下载时间优先的自适应BitTorrent激励协议AIPS.文中首先基于Markov模型对BitTorrent现有激励机制的效果给出了定量分析,分析了激励机制下的文件传输结构,并用概率分析方法给出了该传输结构下最小化文件下载时间的条件.应用分析结果文中定义了一个以缩短文件下载时间为效用的博弈,在该博弈达到Nash平衡时各节点采用的策略就是激励协议AIPS.模拟实验表明文中提出的AIPS较现有的BitTorrent激励协议能明显提高文件共享系统性能,提高文件下载效率.     

一种定量评估复杂网络节点重要度的算法

提出一种定量评估复杂网络节点重要度的算法,定义虚拟的理想“核心节点”,将灰色关联度作为测度,评价网络中每个节点和理想“核心节点”的关联度,关联度越高,节点在网络中的重要性越大。仿真结果表明,与同类算法相比,该算法运算速度更快,适用于评价大型复杂网络中节点的重要度。     

BitTorrent模型的信任机制研究

BitTorrent下载模型得到普遍的应用,但节点缺乏控制,存在虚假资源等安全性问题。针对BitTorrent下载模型的特点,分析了BitTorrent模型的安全问题所在及已有的BitTorrent安全防范机制,结合P2P技术中的信任模型思想,改进BitTorrent安全机制,优化文件的拆分和整合算法,提高下载资源的成功率。仿真实验表明,与传统BitTorrent下载模型相比,改进后的模型能较快发现网络中的虚假资源,具有更高的下载成功率。     

一种基于虚拟力的移动传感器网络再部署算法

针对移动传感器网络中,数据的大规模定向传输的实际特性,提出一种基于虚拟力的移动传感器网络节点精确部署算法（VFPDA）。通过引入“中继节点簇”的概念,在源节点到目的节点之间生成多条虚拟“引力线”,这些引力线相互衔接从而构成一条虚拟的通路,然后吸引周围的移动节点向其靠拢,在“引力线”所产生的引力以及节点之间的斥力等作用力的共同作用下,构建一条从源节点到目的节点的通路。实验仿真验证了该算法的有效性。     

一种适用于嵌入式系统的P2P下载模型*

提出了一种基于BitTorrent协议的嵌入式BT下载模型。该模型对BitTorrent思想进行了深化和应用,针对于快速BT网络,在嵌入式系统应用中进行了改进,有效提高了下载速度,甚至可以在嵌入式系统上快速下载多媒体视频文件进行播放。     

基于HEED的无线传感器网络负载均衡路由算法

针对无线传感器网络存在的“漏斗效应”以及“热点”问题, 在HEED的基础上通过改进簇间路由协议,提出一种负载均衡路由算法HEED-LEDP。该算法通过在簇头节点间构建负载均衡树,保证网络中数据流量均匀分布于各个通信路径,避免大量数据汇集到少数路径上造成的“热点”问题;同时该算法提出一种应用于单条通信路径的轮换转发策略,将一条路径上数据转发的负担分布到所有节点中,从根本上消除“漏斗效应”。实验结果证明HEED-LEDP路由算法能有效避免网络中局部节点过早死亡的情况,提高网络寿命和能量使用效率。     

基于节点生存时间的WSN节能路由算法

针对无线传感器网络寿命受节点能量制约的问题,提出基于节点生存时间的节能算法。根据节点接收、发送和处理数据后的剩余能量,给出节能算法中“轮”的概念,以已有阈值函数为基础进行改进,簇首选举利用改进阈值函数得到该簇最佳簇首节点,从而延长节点生存时间,使簇首以较低能耗延长其轮回周期。仿真结果表明,该算法能有效降低节点死亡速度,延长网络寿命。     

BitTorrent协议中激励机制的设计与分析

激励机制是BitTorrent协议的重要组成部分,它鼓励用户上传以获得更快的下栽速度,并提高系统整体性能。原有BitTorrent激励机制存在大量的搭便车节点,极大影响了激励效果,因此提出了一种新的激励策略,并给出了具体的实施算法。对应用这种新激励机制的BitTorrent协议进行了详细的着色Petri网（colored Petri nets,CPN）形式化建模分析,在验证模型正确性的基础上,进行基于模型模拟的性能分析实验,结果表明该激励机制能够严格限制搭便车节点行为,达到更好的激励效果。     

Performance analysis and improvement for BitTorrent‐like file sharing systems

In this paper, we present a simple mathematical model for studying the performance of the BitTorrent ( http://www.bittorrent.com ) file sharing system. We are especially interested in the distribution of peers in different states of the download job progress. With the model we find that the distribution of the download peers follows an asymmetric U‐shaped curve under the stable state, due to BitTorrent's unchoking strategies. In addition, we find that the seeds' departure rate and the download peers' abort rate will influence the peer distribution in different ways notably. We also analyze the content availability under the dying process of the BitTorrent file sharing system. We find that the system's stability deteriorates with decreasing and unevenly distributed online peers, and BitTorrent's built‐in ‘tit‐for‐tat’ unchoking strategy could not help to preserve the integrity of the file among the download peers. We propose an innovative ‘tit‐for‐tat’ unchoking strategy which enables more peers to finish the download job and prolongs the system's lifetime. By playing our innovative strategy, download peers could cooperate to improve the stability of the system by making a trade‐off between the current downloading rate and the future service availability. Finally, experimental results are presented to validate our analytical results and support our proposals. Copyright © 2007 John Wiley & Sons, Ltd.     

Measurement,modeling and enhancement of BitTorrent-based VoD system

BitTorrent is one of the most popular Peer-to-Peer (P2P) applications for file sharing over the Internet. Video files take up a large proportion of space among the BitTorrent shared files. Recently, BitTorrent has attracted researchers’ interests, as an alternative method of providing video on demand (VoD) service. In this paper, we concentrate on enabling BitTorrent to support VoD service in existing swarms while maintaining the download efficiency of file-sharing users. We first examine the content properties of the BitTorrent system to explore the demands and challenges of VoD service in BitTorrent swarms. The efficiency of BitTorrent for various piece selection policies is then compared through measurement on PlanetLab. We also use an optimization mathematical model to analyze the hybrid BitTorrent system in which downloading peers and streaming peers coexist. Both measurement results and model analysis indicate the problem of system efficiency decline in the BitTorrent-based VoD systems, in comparison with the original BitTorrent file-sharing system. Our proposed approach, unlike existing strategies that are limited to changing the piece selection policy to allow BitTorrent to support streaming services, modifies both piece and peer selection policies to provide a “streaming while downloading” service in the BitTorrent system with downloading peers. For the peer selection policy, a CAP (Closest-Ahead Peers) method is applied to make better use of the peers’ upload bandwidths. For the piece selection policy, a sliding window-based hybrid method that combines the rarest-first policy with the sequential policy is proposed. To demonstrate the performance of our proposed approach, an evaluation is made using various metrics on PlanetLab. The results show that our proposed method has higher throughput and better streaming continuity than the sequential policy and BiToS.     

Free-Riding on BitTorrent-Like Peer-to-Peer File Sharing Systems: Modeling Analysis and Improvement

BitTorrent has emerged as a very popular peer-to-peer file sharing system, which uses an embedded set of incentive mechanisms to encourage contribution and prevent free-riding. However, the capability BitTorrent has of preventing free-riding needs further study. In this paper, we present a fluid model with two different classes of peers to capture the effect of free-riding on BitTorrent-like systems. With the model, we find that BitTorrent's incentive mechanism is successful in preventing free-riding in a system without seeds, but may not succeed in producing a disincentive for free-riding in a system with a high number of seeds. The reason for this is that BitTorrent does not employ any effective mechanisms for seeds to effectively guard against freeriding. Therefore, we propose a seed bandwidth allocation strategy for the BitTorrent system to reduce the effect of seeds on free-riding. Finally, simulation results are given, which validate what we have found in our analysis and demonstrate the effectiveness of the proposed strategy.     

Securing BitTorrent using a new reputation-based trust management system

Nowadays, BitTorrent as a means of sharing files has become highly popular among internet users. However, due to the open nature of BitTorrent protocol and lack of any security mechanism, number of attacks against BitTorrent has significantly increased. Sybil, Collusion, Lying-Piece, Fake-Block, and Chatty-Peer are attack types which have been considered in this paper to secure BitTorrent against them. These attacks can decrease the download performance of BitTorrent clients considerably. In this paper a new reputation based trust management system to cover aforementioned attack types is presented. The proposed approach calculates a local score at peers and a global score at the tracker for each peer. First, peers are sorted according to their cumulative score at the tracker and then top 10 % of these peers are used to determine other peers global score. These local and global scores are used to find attackers. In addition, a novel formula has been utilized to calculate peers local score. Using the global score concept makes our mechanism robust and swift to detect collusion attack which has not been considered in most of similar previous works. In order to evaluate the effectiveness of the proposed system, several simulation and real experiments in the Emulab testbed were performed. The outcomes indicate that our method is highly effective in detection of rogue peers and Free-Riders; moreover, performance of honest peers has significantly improved.     

Optimizing BitTorrent-like peer-to-peer systems in the presence of network address translation devices

BitTorrent nowadays is one of the most important peer-to-peer (P2P) file-sharing applications on the Internet, and it has also inspired many other P2P applications such as live or on-demand video streaming services. On the other hand, Network Address Translation (NAT) has become pervasive in almost all networking scenarios, from residential Internet access to enterprise networks. Despite the effort of NAT traversal, it is still very likely that P2P applications cannot receive incoming connection requests properly if they are behind NAT. To quantify the performance impact of NAT on BitTorrent-like P2P systems, we have created and validated a detailed but tractable mathematical model. We have also proposed and briefly examined a simple but effective strategy to mitigate the negative impact on NAT peers. In this paper, we further extend our model to cover the proposed biased optimistic unchoke strategy, and optimize the system performance in terms of both average peer download time and system finish time. We also create a new, faster Java-based BitTorrent simulator, which is used to validate the extended model and show the optimality of the proposed strategy and its boundary conditions.