A novel recursive shared segment protection algorithm in survivable WDM networks

This paper investigates the problem of dynamic survivable routing for shared segment protection in mesh Wavelength-Division-Multiplexing (WDM) optical networks. We propose a heuristic algorithm, named Recursive Shared Segment Protection (RSSP), to introduce a more flexible way to partition the working path into segments and compute the corresponding backup segments. In RSSP, the working segments cannot be determined before the backup segments are found, we adopt a recursive process to compute the backup segments one by one and then choose an optimized way to partition the working path. The calculations of every neighbor working segment and its backup segment are connected with each other. We constrain the hop count for each backup segment to insure the short failure recovery time and control the bandwidth resource utilization. Compared with the Share Path Protection (SPP), RSSP can achieve much shorter failure recovery time with a little sacrifice in bandwidth resource utilization and RSSP can also perform better compromise between the failure recovery time and the bandwidth resource utilization than the Equal-Length Segment Protection (ELSP) algorithm. We evaluate the effectiveness of RSSP and the results are found to be promising.     

Loop-free traffic engineering with path protection in MPLS VPNs

In this paper we present a new approach for VPN (virtual private network) traffic engineering with path protection in Multiprotocol Label Switching networks carrying QoS and best effort traffic. Our approach eliminates the path cycles, a problem often encountered in link-based traffic engineering methods. It also allows for control of the maximum path length and the size of the label space in each label switch router. We consider off-line computation of the working and backup paths using a link-based approach. Two cases of 1 + 1 and 1:1 path protection are considered. Numerical results are presented to show the efficacy of the algorithm in calculating link-disjoint and node-disjoint primary and backup paths for the QoS traffic.     

基于优化链路权值的域内路由保护方案

目前,互联网部署的域内链路状态路由协议,如开放最短路径优先(Open Shortest Path First,OSPF)和中间系统到中间系统(Intermediate System-to-Intermediate System,IS-IS),采用被动恢复方案应对网络故障。随着网络的发展,大量的实时应用部署在互联网上,OSPF的收敛时间无法满足这些实时应用对收敛时间的需求。因此,学术界和工业界提出采用路由保护方案来应对网路中出现的故障。然而,已有的路由保护方案存在两个方面的问题:1)默认路径和备份路径的交叉度较高,如LFA；2)为了计算两条交叉度低的路径,对默认路径加以限制,即默认路径不采用最短路径,如Color Tree。为了解决上述两个问题,首先将上述问题归结为整数规划模型,接着利用启发式方法计算近似最优解,最后在实际网络和模拟网络中对所提算法进行了大量实验。实验结果表明,所提算法可以降低默认路径和备份路径的交叉度,极大地提高网络的可用性。     

Two-stage Resampling for Bidirectional Path Tracing with Multiple Light Sub-paths

Recent advances in bidirectional path tracing (BPT) reveal that the use of multiple light sub-paths and the resampling of a small number of these can improve the efficiency of BPT. By increasing the number of pre-sampled light sub-paths, the possibility of generating light paths that provide large contributions can be better explored and this can alleviate the correlation of light paths due to the reuse of pre-sampled light sub-paths by all eye sub-paths. The increased number of pre-sampled light subpaths, however, also incurs a high computational cost. In this paper, we propose a two-stage resampling method for BPT to efficiently handle a large number of pre-sampled light sub-paths. We also derive a weighting function that can treat the changes in path probability due to the two-stage resampling. Our method can handle a two orders of magnitude larger number of presampled light sub-paths than previous methods in equal-time rendering, resulting in stable and better noise reduction than state-of-the-art methods.     

一种基于MPLS网络的快速故障恢复算法

现有的MPLS故障恢复方案存在不同的性能问题:Makam方案需要提前建立备份路径,浪费了大量网络资源;简单动态方案动态建立备份路径,资源利用率高,但是需要等待路由表收敛,恢复时间长,造成大量丢包.针对这些不足,提出了一种基于MPLS网络的快速故障恢复算法MBFR.MBFR算法在故障发生以后建立备份路径,但是不需要等待路由表收敛,只需根据PIL中信源树和当前故障信息就可以快速计算出备份路径,既不浪费网络资源,又缩小了恢复时间.仿真实验结果验证了MBFR算法的优越性.     

A new algorithm based on auxiliary virtual topology for sub-path protection in WDM optical networks

This paper proposes a new survivable algorithm named sub-path protection based on auxiliary virtual topology (SPAVT) to tolerate the single-link failure in WDM optical networks. First, according to the protection-switching time constraint, SPAVT searches multiple pairs of primary and backup paths for each node pair in the network by the off-line manner, and then map these paths to the virtual topology. When a connection request arrives, SPAVT only needs to run one time of the Dijkstra’s algorithm to search a virtual route in virtual topology, where the route may consist of multiple pairs of sub-paths, to meet the protection-switching time constraint. Then, according to the shared resources policy, SPAVT chooses an optimal pair of sub-paths. Simulation results show that SPAVT has smaller blocking probability and lower time complexity than conventional algorithms.     

DC规则的高效实现方法

被动恢复方法应对网络故障的恢复时间较长,无法满足实时应用对网络时延和丢包率的要求。因此,路由器厂商普遍采用DC规则来处理网络中的故障。然而,已有的实现DC规则算法的时间复杂度普遍较高,并且随着网络节点平均度的增加而增加。因此,研究了如何降低实现DC规则的复杂度,提出了一种高效的DC实现方法（efficient DC implementation scheme,EDCS）。首先对DC规则进行了扩展,然后在构造最短路径树的过程中实现扩展DC规则,最后从理论上分析了算法的时间复杂度。实验结果表明,EDCS不仅具有较小的计算开销,并且可以计算出所有符合DC规则的备份下一跳。     

基于基本回路的MPLS网络重路由故障恢复机制

研究MPLS网络中的重路由故障恢复机制,提出一种新的计算备用路径的方法,将备用路径的计算分为预处理和在线计算2个过程,给出一种基于基本回路的重路由故障恢复机制(FC-R)。仿真分析表明,FC-R恢复时间较短,可以对抗节点故障或链路故障,大大缩短在线计算时间,减轻节点负担,能够得到性能较优的备用路径,进一步节省网络资源。     

Using Shared Risk Link Groups to enhance backup path computation

To cope quickly with all types of failure risks (link, node and Shared Risk Link Group (SRLG)), each router detecting a failure on an outgoing interface activates locally all the backup paths protecting the primary paths which traverse the failed interface. With the observation that upon a SRLG failure, some active backup paths are inoperative and do not really participate to the recovery (since they do not receive any traffic flow), we propose a new algorithm (SRLG structure exploitation algorithm or SSEA) exploiting the SRLG structures to enhance the admission control and improve the protection rate.With our algorithm, more flexibility is provided for the backup path selection since a backup path which protects against the failure of a link belonging to a SRLG does not systematically bypass all the links of that SRLG. Moreover, our algorithm permits to save more bandwidth because it does not allocate the bandwidth for the inoperative backup paths even if they are activated.Simulations show that our algorithm SSEA decreases the ratio of rejected backup paths and, it reduces in distributed environments the average number of messages sent to manage the bandwidth information necessary for the backup path computation.     

Delay-SRLG constrained,backup-shared path protection in WDM networks with sleep scheduling

This paper addresses the Delay-Shared Risk Link Groups (SRLG) constrained path protection problem in green WDM networks with sleep scheduling, and presents a Green Delay-SRLG Constrained Protection (GDSCP) approach. In order to balance the QoS (delay, SRLG reliability, etc.) and energy consumption, the path search algorithm in GDSCP adopts different principles in the search of the primary and backup paths. The choice of the primary path is optimal for the end-to-end delay while minimizing the node awaking to save energy. When necessary, the rarely used backup paths are allowed to go through more sleeping nodes that lead to potential node awaking to ensure the disjoint degree, and thus increase the SRLG reliability of the combined path. Besides the traditional wavelength sharing between backup paths, our approach further encourages paths of different connections to wake up common sleeping nodes to increase the utilization of the reserved node awaking and thus reduce the demand for the new node-state switching in the network. Comparing to the traditional energy-aware schemes, simulations show promising results that GDSCP can obtain significant improvement in terms of increasing the sleeping percentage of the network and reducing the number of node-state switching without sacrificing the performance of blocking rate.     

Dynamic Path Management with Resilience Constraints under Multiple Link Failures in MPLS/GMPLS Networks

Most previous research on MPLS/GMPLS recovery management has focused on efficient routing or signaling methods from single failures. However, multiple simultaneous failures may occur in large-scale complex virtual paths of MPLS/GMPLS networks. In this paper, we present a dynamic MPLS/GMPLS path management strategy in which the path recovery mechanism can rapidly find an optimal backup path which satisfies the resilience constraints under multiple link failure occurrences. We derived the conditions to test the existence of resilience-guaranteed backup path, and developed a decomposition theorem and backup path construction algorithm for the fast restoration of resilience-guaranteed backup paths, for the primary path with an arbitrary configuration. Finally, simulation results are presented to evaluate the performance of the proposed approach.     

灾害救援实时最快通行路径算法

灾害救援需要物资人力的快速运输,而突发灾害常会影响到道路的通行状态,研究道路网络动态变化情况下救援车辆的实时最快通行路径算法,具有重要的经济和社会意义.针对灾害发生后道路状况多变突变的情况,提出一种实时最快通行路径求解算法ARFTP(Algorithm of Real-time Fastest Traffic Path),将结点进行分类筛选,依据相应准则进行运算,减少了需要重新计算的结点和路径数量.当车辆行驶在原定救援最快通行路径上时,实时收到路段变化信息,根据ARFTP求解策略可快速求出新的最快通行路径.通过仿真验证了算法的有效性和效率,对提高灾害救援运输效率具有一定的意义.     

A non-retraction path planning approach for extrusion-based additive manufacturing

Notwithstanding the widespread use and large number of advantages over traditional subtractive manufacturing techniques, the application of additive manufacturing technologies is currently limited by the undesirable fabricating efficiency, which has attracted attentions from a wide range of areas, such as fabrication method, material improvement, and algorithm optimization. As a critical step in the process planning of additive manufacturing, path planning plays a significant role in affecting the build time by means of determining the paths for the printing head's movement. So a novel path filling pattern for the deposition of extrusion–based additive manufacturing is developed in this paper, mainly to avoid the retraction during the deposition process, and hence the time moving along these retracting paths can be saved and the discontinuous deposition can be avoided as well. On the basis of analysis and discussion of the reason behind the occurrence of retraction in the deposition process, a path planning strategy called “go and back” is presented to avoid the retraction issue. The “go and back” strategy can be adopted to generate a continuous extruder path for simple areas with the start point being connected to the end point. So a sliced layer can be decomposed into several simple areas and the sub-paths for each area are generated based on the proposed strategy. All of these obtainable sub-paths can be connected into a continuous path with proper selection of the start point. By doing this, separated sub-paths are joined with each other to decrease the number of the startup and shutdown process for the extruder, which is beneficial for the enhancement of the deposition quality and the efficiency. Additionally, some methodologies are proposed to further optimize the generated non-retraction paths. At last, several cases are used to test and verify the developed methodology and the comparisons with conventional path filling patterns are conducted. The results show that the proposed approach can effectively reduce the retraction motions and is especially beneficial for the high efficient additive manufacturing without compromise on the part resistance.     

面向串扰时延效应的时序分析方法及在集成电路测试中的应用

随着特征尺寸进入纳米尺度,相邻连线之间的电容耦合对电路时序的影响越来越大,并可能使得电路在运行时失效.准确和快速地估计电路中的串扰效应影响,找到电路中潜在的串扰时延故障目标,并针对这些故障进行测试是非常必要的.文中提出了一种基于通路的考虑多串扰引起的时延效应的静态时序分析方法,该方法通过同时考虑临界通路及为其所有相关侵略线传播信号的子通路来分析多串扰耦合效应.该方法引入了新的数据结构"跳变图"来记录所有可能的信号跳变时间,能够精确地找到潜在的串扰噪声源,并在考虑串扰时延的情况下有效找到临界通路及引起其最大串扰减速效应的侵略子通路集.这种方法可以通过控制跳变图中时间槽的大小来平衡计算精度和运行时间.最后,文中介绍了在基于精确源串扰通路时延故障模型的测试技术中,该静态时序分析方法在耦合线对选择和故障敏化中的应用.针对ISCAS89电路的实验结果显示,文中提出的技术能够适应于大电路的串扰效应分析和测试,并且具有可接受的运行时间.     

基于拥塞及内存感知的SD-WAN故障恢复

在软件定义广域网(SD-WAN)中, 链路故障会导致大量丢包, 严重时会引起部分网络瘫痪. 现有的流量工程方法通过在数据平面提前安装备份路径能够加快故障恢复过程, 但在资源受限的情况下难以适应各种网络故障情况, 从而使恢复后的网络性能下降. 为了保证网络在故障恢复之后的性能并减少备份资源的消耗, 本文提出一种基于拥塞及内存感知的主动式故障恢复方案(CAMA), 不仅能够将受影响数据流进行快速重定向, 还能实现负载均衡避免恢复后潜在的链路拥塞. 实验结果表明, 与已有方案相比, CAMA能有效利用备份资源, 在负载均衡上有较好的性能, 且仅需少量备份规则即可覆盖所有单链路故障情况.     

Joint coverage and link utilization for fast IP local protection

To mitigate the impact of failures, many IP Fast Local Recovery (IPFLR) schemes have been proposed to reroute traffic in the events of failures. However, the existing IPFLR schemes either aimed to find the alternate backup routes to protect failures, or focused on balancing the traffic load routed on the backup routes. Furthermore, in Internet, flows are often managed by shortest path routing, and therefore purely determining the backup routing paths is not sufficient in protecting the error-prone networks. In this paper, we propose a Simulated Annealing based Load balancing and Protection (SALP) scheme to determine link weights for balancing link utilization in the non-failure state and simultaneously construct backup routing tables for protecting any single link failure in IP networks. In our proposed scheme, the two most significant issues, (1) load balancing and (2) coverage, are jointly considered to recover the network operation from single link failures. In the proposed scheme, upon a failure, only the nodes adjacent to a failure are activated to divert affected traffic to backup paths without disturbing regular traffic. Numerical results delineate that the proposed scheme achieves high coverage rate and load balancing at the expense of slightly increasing the entries of backup routing table.     

Loop-free alternates and not-via addresses: A proper combination for IP fast reroute?

The IETF currently discusses fast reroute mechanisms for IP networks (IP FRR). IP FRR accelerates the recovery in case of network element failures and avoids micro-loops during re-convergence. Several mechanisms are proposed. Loop-free alternates (LFAs) are simple but cannot cover all single link and node failures. Not-via addresses can protect against these failures but are more complex, in particular, they use tunneling techniques to deviate backup traffic. In the IETF it has been proposed to combine both mechanisms to merge their advantages: simplicity and full failure coverage.This work analyzes LFAs and classifies them according to their abilities. We qualitatively compare LFAs and not-via addresses and develop a concept for their combined application to achieve 100% single failure coverage, while using simple LFAs wherever possible. The applicability of existing LFAs depends on the resilience requirements of the network. We study the backup path length and the link utilization for both IP FRR methods and quantify the decapsulation load and the increase of the routing table size caused by not-via addresses. We conclude that the combined usage of both methods has no advantage compared to the application of not-via addresses only.     

Multicast protection scheme in survivable WDM optical networks

The protection design is a key issue in survivable wavelength division multiplexing (WDM) optical networks. Most researches focused on protecting unicast traffic against the failure of a single network component such as a link or a node. In this paper, we investigate the protection scheme for multicast traffic in meshed WDM optical networks under dual-link failure consideration, and propose a novel protection algorithm called shared segment protection with reprovisioning (SSPR). Through dynamically adjusting link-cost according to the current network state, SSPR establishes a primary light-tree and corresponding link-disjoint backup segments for each multicast connection request. A backup segment can efficiently share wavelength capacity of its working tree or the common resource of other backup segments. Capacity reprovisioning establishes new segments for the vulnerable connections after a link failure and tolerates following link failures. The simulation results show that SSPR not only can make good use of wavelength resources and protect multicast sessions against any single-link failure, but also can greatly improve the traffic restorability in the event of dual-link breakdown.     

An efficient critical protection scheme for intra-domain routing using link characteristics

In recent years, there are substantial demands to reduce packet loss on the Internet. Among the proposed schemes, finding backup paths in advance is considered to be an effective method to reduce the reaction time. Very commonly, a backup path is chosen to be the most disjoint path from the primary path, or on the network level, a backup path is computed for each link (e.g., IPFRR). The validity of this straightforward choice is based on two things. The first thing is all the links may have the equal likelihood to fail; the second thing is, facing the high protection requirement today, it just looks weird to have links not protected or to share links between the primary and backup paths. Nevertheless, many studies have confirmed that the individual vulnerability of the links on the Internet is far from being equal. In addition, we have observed that full protection schemes (In this paper, full protection schemes means schemes (1) in which backup path is a most disjoint path from the primary path; or (2) which compute backup path for each link.) may introduce high cost (e.g., computation).In this paper, we argue that such approaches may not be cost-efficient and therefore propose a novel critical protection scheme based on link failure characteristics. Firstly, we analyze the link failure characteristics based on real world traces of CERNET2 (China Education and Research NETwork 2). The analysis results clearly show that the failure probabilities of the links in CERNET2 backbone are heavy-tailed, i.e., a small set of links causing most of the failures. Based on this observation, we find out two key parameters which strongly impact link criticality and propose a critical protection scheme for both single link failure situation and multi-link failure situation. We carefully analyze the implementation details and overhead for backup path schemes of the Internet today; the problem is formulated as an optimization problem to guarantee the routing performance and minimize the backup cost. This cost is special as it involves computational overhead. Based on this, we propose a novel Critical Protection Algorithm which is fast itself for both the single link failure and the multi-link failure versions. A comprehensive set of evaluations with randomly generated topologies, real world topologies and the real traces from CERNET2, shows that our scheme gains significant achievement over full protection in both single link failure situation and multi-link failure situation. It costs only about 30–60% of the full protection cost when the network relative availability increment is 90% of the full protection scheme.     

Quality of service based resource allocation for scheduled lightpath demands

In this paper, we consider resource allocation under the scheduled traffic model, where the setup and teardown times of scheduled demands are known in advance. A number of integer linear program (ILP) solutions for this problem have been presented in the literature. Most of these ILPs minimize the number of wavelength links. A more appropriate metric, for wavelength routed, all-optical networks, is to minimize the congestion (the maximum number of lightpaths on a single fiber link) of the network. We present a new ILP formulation for routing and wavelength allocation, under the scheduled traffic model that minimizes the congestion of the network. We propose two levels of service, where idle backup resources can be used to carry low-priority traffic, under fault-free conditions. When a fault occurs, and resources for a backup path need to be reclaimed, any low-priority traffic on the affected channels is dropped. The results demonstrate that this can lead to significant improvements over single service level models. We also show that the complexity of our formulation (in terms of the number of integer variables) is lower, even compared to existing approaches, which only allow a single level of service. Therefore, we are able to generate optimal solutions for practical networks within a reasonable amount of time, whereas existing formulations often do not find the optimal solution even after 2 h. Finally, we present a simple and fast heuristic that can quickly generate good solutions for much larger networks.