新的基于MPH的时延约束Steiner树算法

为了在时延约束条件下进一步优化多播树代价并降低算法的复杂度,研究了时延受限的Steiner树问题。在DCMPH算法的基础上,通过改进节点的搜索路径,提出了一种新的基于MPH的时延约束Steiner树算法。该算法中每个目的节点通过最小代价路径加入当前多播树;若时延不满足要求,则通过合并最小时延树进而产生一个满足时延约束的最小代价多播树。仿真实验表明,新算法在性能、空间复杂度方面均优于DCMPH算法。     

基于加权节点的Steiner树启发式算法

Steiner最小树问题是一个NP完全问题,被广泛应用在通信网络中点到多点的路由选择。为了实现更多链路的共享,减少所求Steiner树的费用,提出了一种基于加权节点求解Steiner树的启发式(NWMPH)算法。该算法构造了非正则点的权值公式,给每一个非正则点赋权值,根据权值对链路的费用进行修正,通过修正费用最短路径依次把所有的正则点连接起来,得到包含所有正则点的最小树。对STEINLIB标准数据集中的部分数据进行计算,结果表明: NWMPH算法与MPH算法所用时间基本相同,得到的Steiner树费用优于MPH算法;NWMPH算法比KBMPH算法所用时间少,得到的Steiner树费用绝大多数优于KBMPH算法。     

多媒体通信中带度约束的多播路由算法

随着多媒体业务的发展,多播技术应用日益广泛,多播路由是要寻找连接源节点和一组目的节点的一棵多播树,这个问题在数学上归结为Steiner树问题,它是一个NPC问题。在实际网络中,网络节点具备不同的多播能力,有些节点不支持多播,有些节点支持多播,但为了保证网络速度和节点负载平衡,支持多播的节点要限制其复制信息的数量,即节点的多播能力受限。在这种情况下,寻找多播树变得更加困难,该文用节点的约束来表示敏个节点具备的多播能力,节点多播能力受限情况下的多播路由问题被称为带度约束的多播路由问题,其仍是一个NPC问题。该文提出了一种求解带度的约束多播路由问题的双层遗传算法。算法的基本思想是最优多播树应是一棵满足度约束的最小生成树,因此问题的关键在于如何找到包括在最优生成树中的Steiner节点。遗传算法 采用二进制编码方式,内层算法用于求解满足度约束的最小生成树;外层算法进行全局搜索。该文将算法在稀疏图上进行实验,为了更好地模拟真实网络,稀疏图中每个节点具有不同的多播能力,并且多播目的节点数目相比于网络节点数要小。实验对算法进行了三方面比较：（1）解的质量;（2）计算时间;（3）算法的收敛性。实验结果表明,文中提出的遗传算法能够找到费用较小的多播树,但是当网络规模增大时,算法的求解时间也较长。     

Ad hoc网络时延受限的Steiner树启发式算法

针对Ad hoc网络时延受限的Steiner树问题,设计一个分布式的快速启发式算法DCST,该算法通过对网络中节点进行标号,并根据标号修改节点间的关联关系,建立一棵时延受限的Steiner树。在网络节点保持时间同步的前提下,算法的时间复杂度为O(n)。与现有经典的Steiner树算法相比,该算法具有明显优势。     

基于遗传算法的时延受限多播路由研究

该文探讨了包交换计算机网络中,具有端到端时延限制的多播路由问题。提出了一种基于遗传算法的多播路由优化算法,利用该算法可以实现在给定网络和多播需求的情况下,寻找费用最小的多播路由树,使该树覆盖所有的多播目的节点,并使网络费用达到最小。     

基于蚁群算法的QoS多播路由优化策略

对QoS多播路由和约束最小Steiner多播树进行了分析,提出了基于蚁群算法搜索约束最小Steiner多播树的ACMC算法,并与DDMC算法进行了实验比较.结果表明,在同样环境和多播组规模的条件下,ACMC算法花费的网络代价小于DDMC算法,从而验证了ACMC算法的有效性和可行性.     

欧氏平面快速k-瓶颈斯坦纳树近似算法

瓶颈k-Steiner树问题描述如下：给定n个点和一个正整数k,寻找一棵Steiner树用至多k个Steiner点将n个点连接起来,使得此Steiner树的最长边最短。L. Wang和D.-Z. Du证明了适用于欧几里得平面瓶颈斯坦纳树算法的近似性能比为2,并且给出了一个适用于该问题时间复杂度为(nlogn+kn)的算法,在欧几里得平面上和近似性能比为2的前提下,通过引入最大堆和斐波那契堆分别对该算法进行优化,优化后算法的时间复杂度分别达到(nlogn+klogn)和(nlogn+k),优化后的算法在现实中可以更好地应用。     

基于Tabu搜索的QoS多播路由快速优化算法

QoS多播路由算法的核心问题是建立满足QoS约束的多播树,这就是计算机网络中著名的受约束最小Steiner树问题,是一个NP完全问题.目前已有的启发式算法的时间复杂度大,不能获得最优解.提出了一个基于Tabu搜索的QoS多播路由选择快速优化算法,它选择延迟与带宽约束为QoS参数,利用Tabu搜索的集中性与广泛性并存的优点,在提高搜索速度的同时可以更加接近最优解.仿真结果表明:该算法具有快速、易实施等特点,更加适合在组规模比较大的情况下应用.     

网络可存活生成树的快速恢复算法

如何应对网络链接失效是具有挑战性的问题之一,通常采用包含两棵生成树的可存活连接来预防链接失效。由于网络数据传输速率的高速增长,当两棵生成树的共享链接失效时,可存活连接中的生成树将全部失效。针对可存活连接中共享链接的失效提出了一种快速恢复算法,该算法通过搜索失效链接的可替换链接集,将失效概率最小的链接加入原可存活连接中的生成树,生成新的可存活连接。实验结果表明,该算法能够在显著降低恢复时间和时间复杂度的情形下,同时保证可存活连接的存活度接近当前网络的最优存活度。当网络节点数在10～100变化时,提出的算法比现有算法在恢复时间上的平均优化高达34.42%,同时在存活度上的误差不超过1%。     

欧式平面上瓶颈Steiner树问题的一个近似算法

近些年来，Steiner树问题在理论和应用上都引起了极大的关注，尤其在日渐成熟的近似算法设计理论方面，该问题占有一定的中心地位。给定赋权连通图G=(V，E，W)及顶点子集S包含V(S中顶点称为terminals)，传统的Steiner树问题要求寻找一棵最小的树联接5中的所有顶点，该树可能包含V-S中的顶点(称为Steiner点)。即使图中每条边的权值仅限制为1或2时，传统的Steiner树问题仍然是MAX—SNP Hard。     

Adaptive optimization of join trees for multi-join queries over sensor streams

Data processing applications for sensor streams have to deal with multiple continuous data streams with inputs arriving at highly variable and unpredictable rates from various sources. These applications perform various operations (e.g. filter, aggregate, join, etc.) on incoming data streams in real-time according to predefined queries or rules. Since the data rate and data distribution fluctuate over time, an appropriate join tree for processing join queries must be adaptively maintained in response to dynamic changes to prevent rapid degradation of the system performance. In this paper, we address the problem of finding an optimal join tree that maximizes throughput for sliding window based multi-join queries over continuous data streams and prove its NP-Hardness. We present a dynamic programming algorithm, OptDP, which produces the optimal tree but runs in an exponential time in the number of input streams. We then present a polynomial time greedy algorithm, XGreedyJoin. We tested these algorithms in ARES, an adaptively re-optimizing engine for stream queries, which we developed by extending Jess (Jess is a popular RETE-based, forward chaining rule engine written in java). For almost all instances, trees from XGreedyJoin perform close to the optimal trees from OptDP, and significantly better than common heuristics-based XJoin algorithms.     

Algorithms for delay-constrained low-cost multicast tree construction

With the proliferation of multimedia group applications, the construction of multicast trees satisfying quality of service (QoS) requirements is becoming a problem of prime importance. Multicast groups are usually classified as sparse or pervasive groups depending on the physical distribution of group members. They are also classified based on the temporal characteristics of group membership into static and dynamic groups. In this paper, we propose two algorithms for constructing multicast trees for multimedia group communication in which the members are sparse and static. The proposed algorithms use a constrained distributed unicast routing algorithm for generating low-cost, bandwidth and delay constrained multicast trees. These algorithms have lower message complexity and call setup time due to their nature of iteratively adding paths, rather than edges, to partially constructed trees. We study the performance (in terms of call acceptance rate, call setup time and multicast tree cost) of these algorithms through simulation by comparing them with that of a recently proposed algorithm (V. Kompella, J.C. Pasquale, G.C. Polyzos, Two distributed algorithms for the constrained Steiner tree problem, in: Proc. Comp. Comm. Networking, San Diego, CA, June 1993) for the same problem. The simulation results indicate that the proposed algorithms provide larger call acceptance rates, lower setup times and comparable tree costs.     

Heuristic algorithms for designing self-repairing protection trees in mesh networks

Protection trees have been used in the past for restoring multicast and unicast traffic in networks in various failure scenarios. In this paper we focus on shared self-repairing trees for link protection in unicast mesh networks. Shared protection trees have been proposed as a relatively simple approach that is easy to reconfigure and could provide sub-second restoration times with sub-optimal redundancy requirement. The self-repairing nature of this class of protection trees may make them an attractive option for cases where dynamic changes in network topology or demand may occur. In this paper, we present heuristic algorithms to design a self-repairing protection tree for a given network. We study the restorability performance of shared trees and examine the limitations of such schemes in specific topologies, such as cases where long node chains exist. Using extensive simulations with thousands of randomly generated network graphs. We compare redundancy and average backup path length of shared protection trees with optimal tree designs and non-tree designs. We also apply our algorithms to the problem of designing the protection tree in a pre-designed fixed-capacity network, and study the performance of shared protection trees in this scenario under different network loads and link utilization levels.     

边赋权森林ω-路划分的O(n)算法

ω-路划分问题是路划分问题的一般化,它源于并行计算机系统、计算机网络与分布式控制系统等一类广播通信问题.设置最少的信息源节点,使得在指定的时间内将信息源节点所拥有的信息发送到其余节点,并且保证不同通信线路之间不得相交.从Hamilton路的NP-完全性不难看出,ω-路划分问题属于NP-完全问题.通过构造性证明技术,获得了边赋非负权路径、树和森林的ω-路划分问题的一些性质.分别提出了求解边赋非负权路径和边赋非负权树的ω-路划分问题的线性时间算法,讨论了算法的局部实现技术,详细地分析了这些算法的复杂度.以这两个算法为基础,提出了一个线性时间算法求解边赋非负权森林的ω-路划分问题.所提出的算法直观简明、操作容易,只需要较少的运行时间和较小的存储空间.     

On Maximizing Tree Bandwidth for Topology-Aware Peer-to-Peer Streaming

In recent years, there has been an increasing interest in peer-to-peer (P2P) multimedia streaming. In this paper, we consider constructing a high-bandwidth overlay tree for streaming services. We observe that underlay information such as link connectivity and link bandwidth is important in tree construction, because two seemingly disjoint overlay paths may share common links on the underlay. We hence study how to construct a high-bandwidth overlay tree given the underlay topology. We formulate the problem as building a Maximum Bandwidth Multicast Tree (MBMT) or a Minimum Stress Multicast Tree (MSMT), depending on whether link bandwidth is available or not. We prove that both problems are NP-hard and are not ap-proximable within a factor of (2/3 + epsiv), for any epsiv > 0, unless P = NP. We then present approximation algorithms to address them and analyze the algorithm performance. Furthermore, we discuss some practical issues (e.g., group dynamics, resilience and scalability) in system implementation. We evaluate our algorithms on Internet-like topologies. The results show that our algorithms can achieve high tree bandwidth and low link stress with low penalty in end-to-end delay. Measurement study based on Plan-etLab further confirms this. Our study shows that the knowledge of underlay is important for constructing efficient overlay trees.     

Heuristics-based infeasible path detection for dynamic test data generation

Automated test data generation plays an important part in reducing the cost and increasing the reliability of software testing. However, a challenging problem in path-oriented test data generation is the existence of infeasible program paths, where considerable effort may be wasted in trying to generate input data to traverse the paths. In this paper, we propose a heuristics-based approach to infeasible path detection for dynamic test data generation. Our approach is based on the observation that many infeasible program paths exhibit some common properties. Through realizing these properties in execution traces collected during the test data generation process, infeasible paths can be detected early with high accuracy. Our experiments show that the proposed approach efficiently detects most of the infeasible paths with an average precision of 96.02% and a recall of 100% of all the cases.     

Call control with k rejections

Given a set of connection requests (calls) in a communication network, the call control problem is to accept a subset of the requests and route them along paths in the network such that no edge capacity is violated, with the goal of rejecting as few requests as possible. We investigate the complexity of parameterized versions of this problem, where the number of rejected requests is taken as the parameter. For the variant with pre-determined paths, the problem is fixed-parameter tractable in arbitrary graphs if the link capacities are bounded by a constant, but W[2]-hard if the link capacities are arbitrary. If the paths are not pre-determined, no FPT algorithm can exist even in series-parallel graphs unless . Our main results are new FPT algorithms for call control in tree networks with arbitrary edge capacities and in trees of rings with unit edge capacities in the case that the paths are not pre-determined.     

Optimal multicast routing with quality of service constraints

We consider the problem of optimal multicast routing with Quality of Service constraints motivated by the requirements of interactive continuous media communication, e.g., real-time teleconferencing. We concentrate on distributed algorithms for determining a tree over the network topology, rooted at the source and spanning the intended destinations. Quality of Service requirements for interactive continuous media typically impose constraints on some metric over the individual paths from the source to each destination, usually in the form of an upper bound on the delay. Thus, we focus on the problem of minimizing the cost of the tree while at the same time satisfying a common constraint over individual source-destination paths. We have shown that this problem is intractable, but have also devised centralized polynomial time heuristics that perform well. Here we present distributed algorithms to minimize tree cost while satisfying the constraints on the paths from the source to each destination.     

CMST问题的新算法

本文提出了解决按约束条件求最小代价生成树(简称CMST)问题的两个新算法,即给定结点数N,每个结点的负载,链路的代价及链路的容量后,在符合某些约束条件下,求代价最小的树结构.两个新算法的计算复杂性均为O(N~2).计算结果表明,新算法所得结果的代价低于几个现有算法,而计算复杂性比现有算法小得多.     

后缀树的并行构造算法

后缀树是一种非常重要的数据结构，它在与字符串处理相关的各种领域里有着非常广泛的应用。构造后缀树是应用后缀树解决问题的前提和关键。虽然很多现有的后缀树构造算法都是线性时间和空间的，但是，当被索引的字符串的长度很长时，构造其后缀树所消耗的时间和空间仍将非常巨大，这极大地限制了后缀树的实际应用。而并行技术是解决这一问题的很好途径，因此人们提出了后缀树的并行构造算法。本文对后缀树的三种并行构造算法进行了综述，通过系统的比较和分析，总结出当前存在的问题，并指明了下一步的研究方向。