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.     

Traffic recovery time constrained shared sub-path protection algorithm in survivable WDM networks

With the development of real-time applications, the traffic recovery time, which is defined as the duration between the failure occurrence on the working path and the interruptive traffic has been successfully switched to the backup path, has become the basic Quality-of-Service (QoS) requirement in survivable WDM networks. In this paper, we address the problem of shared sub-path protection with considering the constraint of traffic recovery time and propose a new heuristic algorithm called Traffic recovery time Constrained Shared Sub-Path Protection (TC_SSPP) to compute the working path and the Shared-Risk-Link-Group (SRLG)-disjoint backup sub-paths. The main target of our work is to improve the resource utilization ratio and reduce the blocking probability for dynamic network environment. By properly setting the delay parameter for each link and running the Delay Constrained Shortest Path Algorithm (DCSPA) to compute the backup sub-paths, TC_SSPP can effectively guarantee the traffic recovery time. Simulation results show that the proposed TC_SSPP can outperform the traditional algorithms.     

A new shared-risk link groups (SRLG)-disjoint path provisioning with shared protection in WDM optical networks

This paper addresses the protection problem in WDM optical networks and presents a New Shared-risk link groups (SRLG)-Disjoint Path Provisioning (NSDPP) approach with shared protection to tolerate the single-risk failure. Comparing to previous Shared-risk link groups (SRLG)-Disjoint Path Provisioning (SDPP) approach, NSDPP is able to obtain better performance, because in NSDPP some primary path and backup paths of other primary paths can share the common resources if the corresponding rules can be satisfied. Simulation results are shown to be promising.     

WDM环网保护机制研究

波分复用（WDM）技术是目前发展很快的光传输技术之一。本文研究分析了单向波长通道保护环、双向波长通道保护环、两纤光复用段双向保护环这三种WDM环网保护机制,并对三种环网保护机制进行了比较全面的性能比较,所得出的结论对网络工作者根据不同的网络环境选择正确有效的保护机制起到了一定的理论指导作用。     

Protections for multicast session in WDM optical networks under reliability constraints

In this paper, we investigate protections of multicast session under reliability constraints in WDM optical networks, which is not referred in previous works. All of the papers about protection of multicast session discuss 100% reliability that may be distinct in different users’ requirement. At the beginning of the paper, we discuss the reliability of a tree. Then under reliability constraints, we propose three novel protection algorithms, which are Arc-Disjoint Tree with Different Reliability (ADT_DiR), Partial Tree Protection (PTP) and Choosing Segments Protection (CSP). ADT_DiR finds an arc-disjoint tree if the reliability of primary tree does not meet users’ requirement. PTP finds a small protection tree for some part of primary tree and also meets users’ requirement. CSP divides primary tree into segments first, and then protects some segments that are picked up by a strategy while satisfying users’ requirement. Compared with all protection schemes in other papers, which provide 100% reliability, ADT_DiR, PTP and CSP all decrease the resources needed by backup tree and reduce the failure probability of finding backup trees. The simulation results show that all the three protection schemes decrease the blocking ratio and reduce the protection resources consumed comparing to other protection schemes while meeting users’ requirement under dynamic traffics.     

Providing resiliency for optical grids by exploiting relocation: A dimensioning study based on ILP

Grids use a form of distributed computing to tackle complex computational and data processing problems scientists are presented with today. When designing an (optical) network supporting grids, it is essential that it can overcome single network failures, for which several protection schemes have been devised in the past. In this work, we extend the existing Shared Path protection scheme by incorporating the anycast principle typical of grids: a user typically does not care on what specific server this job gets executed and is merely interested in its timely delivery of results. Therefore, in contrast with Classical Shared Path protection (CSP), we will not necessarily provide a backup path between the source and the original destination. Instead, we allow to relocate the job to another server location if we can thus provide a backup path which comprises less wavelengths than the one CSP would suggest. We assess the bandwidth savings enabled by relocation in a quantitative dimensioning case study on an European and an American network topology, exhibiting substantial savings of the number of required wavelengths (in the order of 11-50%, depending on network topology and server locations). We also investigate how relocation affects the computational load on the execution servers. The case study is based on solving a grid network dimensioning problem: we present Integer Linear Programming (ILP) formulations for both the traditional CSP and the new resilience scheme exploiting relocation (SPR). We also outline a strategy to deal with the anycast principle: assuming we are given just the origins and intensity of job arrivals, we derive a static (source, destination)-based demand matrix. The latter is then used as input to solve the network dimensioning ILP for an optical circuit-switched WDM network.     

低开销混合式保护的高可靠的波分/时分混合无源光网络

针对光纤故障参与保护倒换元器件多、保护资源开销大的问题,提出一种低开销混合式保护的高可靠波分/时分混合无源光网络(WDM/TDM-PON)结构。所提结构在光线路终端(OLT)中新设计了逻辑判定、备份收发以及保护路控制单元,使波分复用(WDM)部分只对故障部分器件执行倒换操作,实现了低开销的集中式保护;在时分复用(TDM)部分采用交叉总线形结构,实现了快速的分布式保护倒换。经分析证明,所提混合式保护结构有效地减少了保护资源开销,对馈入光纤(FF)、配线光纤(DF)和最后一个公里光纤(LMF)故障提供了1.5~2.4 ms恢复时间的快速全保护,且具有良好的扩展性。     

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.     

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.     

A new approach for designing fault-tolerant WDM networks

In recent years, path protection has emerged as a widely accepted technique for designing survivable WDM networks. This approach is attractive, since it is able to provide bandwidth guarantees in the presence of link failures. However, it requires allocating resources for backup lightpaths, which remain idle under normal fault-free conditions. In this paper, we introduce a new approach for designing fault-tolerant WDM networks, based on the concept of survivable routing. Survivable routing of a logical topology ensures that the lightpaths are routed in such a way that a single link failure does not disconnect the network. When a topology is generated using our approach, it is guaranteed to have a survivable routing. We further ensure that the logical topology is able to handle the entire traffic demand after any single link failure. We first present an ILP that optimally designs a survivable logical topology, and then propose a heuristic for larger networks. Experimental results demonstrate that this new approach is able to provide guaranteed bandwidth, and is much more efficient in terms of resource utilization, compared to both dedicated and shared path protection.     

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.     

Routing and wavelength assignment for hypercube in array-based WDM optical networks

Hypercube is one of the most versatile and efficient communication patterns shared by a large number of computational problems. As the number of edges in hypercube grows logarithmically with the size of networks, the complexity of network topologies can be significantly reduced to realize hypercube in optical networks by taking advantage of the parallel transmission characteristic of optical fibers. In this paper, we study the routing and wavelength assignment for realizing hypercube on WDM optical networks including linear arrays and rings with the consideration of communication directions. Specifically, we analyze this problem for both bidirectional and unidirectional hypercubes. For each case, we identify a lower bound on the number of wavelengths required, and design the embedding scheme and wavelength assignment algorithm that uses a provably near-optimal number of wavelengths. In addition, we extend the results to meshes and tori. By our embedding schemes, many algorithms, originally designed based on hypercubes, can be applied to optical networks, and the wavelength requirements can be easily derived using our obtained results.     

一种新的WDM网状网中的动态子通路保护算法

论文研究了业务量疏导WDM网状网中的生存性问题,提出一种新的基于优先级的自适用子通路保护算法(PASPP)。该算法在为子通路寻找保护通路时,高优先级业务可以通过抢占低优先级的业务保护通路来提高其连通率,而同时对低优先级业务也能提供部分保护。仿真结果表明,该算法有较好的性能。     

基于波长选择型可重构光分插复用设备的IP网络的传输长度设计

针对高速大容量波分复用(WDM)网络在多点故障、变更的情况下,运行效率低、维护成本高的问题,提出使用可重构光分插复用设备(ROADM)元件来构建灵活网络.首先,给出了所用的5节点网络配置模型;然后,研究了在动态网络条件下使用ROADM的光网络损耗与传输长度间关系,提出网络传输长度设计流程;其次,基于ROADM搭建一个5节点双向光纤环路实验网络,测量了光损失特性;最后,分析实验结果,得出损耗计算值与光纤实装损耗测量值近似相等(相差0.8 dB),验证了设计的可行性,保证了节点间的可靠传输.     

软交换呼叫服务器中高效备份的方法

软交换呼叫服务器的数据吞吐量十分巨大，影响电信网络通信性能。目前的研究大多局限于通过加大网络传输能力以及采用负荷分担的形式来减轻负荷。该文针对软交换中呼叫服务器的备份过程，改进了备份系统模型，并将提取/恢复函数引入呼叫服务器的备份系统，有效地提高了主备板之间的备份效率，从而提高了呼叫服务器的可靠性。     

An improved scheme for self-healing in ATM networks

Broadband networks based on ATM technology can carry a large volume of data and can support diverse services like audio, video, and data uniformly. The reliability and availability levels provided by such networks should be very high. Self-healing is an elegant concept in this direction to provide highly reliable networks. A self-healing network can detect failures such as link/node failures and reroute the failed connections automatically using distributed control mechanisms. In this paper, we consider link and node failures including the VP terminating nodes unlike Kawamura and Tokizawa (Self-healing in ATM networks based on virtual path concept, IEEE Journal on Selected Areas in Communications 12 (1) (1994) 120–127). We present here an improved scheme for self-healing in ATM networks based on the concept of backup VPs. The problems we address are: (i) self-healing scheme; and (ii) backup VP routing. Two issues are addressed in the self-healing scheme: (i) backup VP activation protocol; and (ii) dynamic backup VP routing. We propose a new backup VP activation protocol which uses a VC packing strategy which allows the fast and prioritized restoration of critical VCs that were carried by failed VPs. We also propose a distributed dynamic backup VP routing algorithm which reduces the resource contention that may occur when multiple source–destination pairs contend for the routes simultaneously. The objective of the backup VP routing problem is to find a backup VP for each of the working VPs so that the cost of providing the backup is minimized. We propose a heuristic based solution for the backup VP routing problem using the concept of minimum cost shortest paths. We conducted simulation experiments to evaluate the performance of the proposed schemes. The results show that the proposed schemes are effective. Comparison of the results with those of the earlier schemes (R. Kawamura, I. Tokizawa, Self-healing in ATM networks based on virtual path concept, IEEE Journal on Selected Areas in Communications 12 (1) (1994) 120–127; C.J. Hou, Design of a fast restoration mechanism for virtual path-based ATM networks, Proceedings of IEEE INFOCOM’97, Kobe, Japan, April 1997) shows that the proposed schemes perform better.     

Cost Efficient Fault Tolerant Design in Mesh Optical Networks with the Load Aware Method

With increasing wavelength capacity, the fault tolerant policy of the mesh optical network becomes much more important. To meet the survivability requirement, a set of cost efficient fault tolerant mechanisms are designed in this paper. The main objective is the minimization of the total number of required wavelengths. Furthermore, based on the method of the Integer Linear Program (ILP), shared-path protection and shared-link protection strategies are achieved, respectively. Moreover, the objective of minimizing the load imbalance is used to ensure the network resources utilized efficiently, and then the wavelength contention can be mitigated by selecting the working path and backup path reasonably. The results show that the survivability of the optical network can be guaranteed in a cost efficient manner.     

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.     

PLR-based heuristic for backup path computation in MPLS networks

To ensure service continuity in networks, local protection pre-configuring the backup paths is preferred to global protection. Under the practical hypothesis of single physical failures in the network, the backup paths which protect against different logical failure risks (node, link and shared risk link group (SRLG)) cannot be active at the same time. Thus, sharing bandwidth between such backup paths is crucial to increase the bandwidth availability.In this article, we focus on the optimal on-line distributed computation of the bandwidth-guaranteed backup paths in MPLS networks. As the requests for connection establishment and release arrive dynamically without knowledge of future arrivals, we choose to use the on-line mode to avoid LSP reconfigurations. We also selected a distributed computation to offer scalability and decrease the LSP setup time. Finally, the optimization of bandwidth utilization can be achieved thanks to the flexibility of the path choice offered by MPLS and to the bandwidth sharing.For a good bandwidth sharing, the backup path computation entities (BPCEs) require the knowledge and maintenance of a great quantity of bandwidth information (e.g. non aggregated link information or per path information) which is undesirable in distributed environments. To get around this problem, we propose here a PLR (point of local repair)-based heuristic (PLRH) which aggregates and noticeably decreases the size of the bandwidth information advertised in the network while offering a high bandwidth sharing. PLRH permits an efficient computation of backup paths. It is scalable, easy to be deployed and balances equitably computations on the network nodes.Simulations show that with the transmission of a small quantity of aggregated information per link, the ratio of rejected backup paths is low and close to the optimum.     

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.