Achieving fast and bandwidth-efficient shared-path protection

Dynamic provisioning of restorable bandwidth guaranteed paths is a challenge in the design of broad-band transport networks, especially next-generation optical networks. A common approach is called (failure-independent) path protection, whereby for every mission-critical active path to be established, a link (or node) disjoint backup path (BP) is also established. To optimize network resource utilization, shared path protection should be adopted, which often allows a new BP to share the bandwidth allocated to some existing BPs. However, it usually leads the backup paths to use too many links, with zero cost in term of additional backup bandwidth, along its route. It will violate the restoration time guarantee. In this paper, we propose novel integer linear programming (ILP) formulations by introducing two parameters (/spl epsi/ and /spl mu/) in both the sharing with complete information (SCI) scheme and the distributed partial information management (DPIM) scheme. Our results show that the proposed ILP formulations can not only improve the network resource utilization effectively, but also keep the BPs as short as possible.     

Routing restorable bandwidth guaranteed connections using maximum 2-route flows

Routing with service restorability is of much importance in Multi-Protocol Label Switched (MPLS) networks, and is a necessity in optical networks. For restoration, each connection has an active path and a link-disjoint backup path. The backup path enables service restoration upon active path failure. For bandwidth efficiency, backups may be shared. This requires that at least the aggregate backup bandwidth used on each link be distributed to nodes performing route computations. If this information is not available, sharing is not possible. Also, one scheme in use for restorability in optical networks is for the sender to transmit simultaneously on the two disjoint paths and for the receiver to choose data from the path with stronger signal. This has the advantage of fast receiver-initiated recovery upon failure but it does not allow backup sharing. In this paper, we consider the problem of efficient dynamic routing of restorable connections when backup sharing is not allowed. Our objective is to be able to route as many connections as possible for one-at-a-time arrivals and no knowledge of future arrivals. Since sharing cannot be used for achieving efficiency, the goal is to achieve efficiency by improved path selection. We show that by using the minimum-interference ideas used for nonrestorable routing, we can develop efficient algorithms that outperform previously proposed algorithms for restorable routing such as routing with the min-hop like objective of finding two disjoint paths with minimum total hop-count. We present two new and efficient algorithms for restorable routing without sharing, and one of them requires only shortest path computations. We demonstrate that both algorithms perform very well in comparison to previously proposed algorithms.     

Path-based routing provisioning with mixed shared protection in WDM mesh networks

In this paper, the authors focus on studying the problem of survivable routing provisioning to prevent single link failure in wavelength-division-multiplexing (WDM) mesh networks, and propose a novel protection scheme called mixed shared path protection (MSPP). With MSPP, the authors define three types of resources: 1) primary resources that can be used by primary paths; 2) spare resources that can be shared by backup paths; and 3) mixed resources that can be shared by both the primary and the backup paths. In the proposed protection scheme, each connection is assigned a primary path and a link disjoint backup path. Differing from pervious protection schemes, MSPP allows some primary paths and backup paths to share the common mixed resources if the corresponding constraints can be satisfied. In this paper, the authors consider three types of path-based protection schemes, i.e., dedicated path protection (DPP), shared path protection (SPP), and MSPP, and evaluate their performance for both the static and the dynamic provisioning problems. Simulation results show that MSPP outperforms DPP and SPP.     

A disjoint path selection scheme with shared risk link groups in GMPLS networks

This letter proposes a disjoint path selection scheme for generalized multi-protocol label switching (GMPLS) networks with shared risk link group (SRLG) constraints. It is called the weighted-SRLG (WSRLG) scheme. It treats the number of SRLG members related to a link as part of the link cost when the k-shortest path algorithm is executed. In WSRLG, a link that has many SRLG members is rarely selected as the shortest path. Simulation results show that WSRLG finds more disjoint paths than the conventional k-shortest path algorithm.     

Dynamic routing of restorable bandwidth-guaranteed tunnels using aggregated network resource usage information

The paper presents new algorithms for dynamic routing of restorable bandwidth-guaranteed paths. We assume that connections are requested one-by-one and there is no prior knowledge of future arrivals. In order to guarantee restorability an alternate link (node) disjoint backup (restoration) path has to be determined, as well as an active path, when the connection is initiated. This joint on-line routing problem is particularly important in optical networks and in MPLS networks for dynamic provisioning of bandwidth-guaranteed or wavelength paths. A simple solution is to find two disjoint paths, but this results in excessive resource usage. Backup path bandwidth usage can be reduced by judicious sharing of backup paths amongst certain active paths while still maintaining restorability. The best sharing performance is achieved if the routing of every path in progress in the network is known to the routing algorithm at the time of a new path setup. We give a new integer programming formulation for this problem. Complete path routing knowledge is a reasonable assumption for a centralized routing algorithm, but is not often desirable, particularly when distributed routing is preferred. We show that a suitably developed algorithm which uses only aggregated information, and not per-path information, is able to perform almost as well as one using complete information. Disseminating this aggregate information is feasible using proposed traffic engineering extensions to routing protocols. We formulate the dynamic restorable bandwidth routing problem in this aggregate information scenario and develop efficient routing algorithms. The performance of our algorithm is close to the complete information bound.     

A novel two-step approach to restorable dynamic QoS routing

Aiming at minimizing the combined bandwidth cost of a pair of disjoint active and backup paths, a popular approach to designing restorable dynamic quality of service (QoS) routing schemes is based on the integer linear programming (ILP) formulation. Owing to the very different natures of active and backup paths, we found this approach problematic. In this paper, we propose an alternative approach, called two-step restorable QoS routing. In the first step, an active path is found using the widest shortest path (WSP) routing. In the second step, the corresponding backup path is determined using one of the three variants of shortest widest path (SWP) routing: basic SWP, approximate SWP or composite SWP. Combining the two steps, three novel two-step routing algorithms, denoted by SBW, SAW, and SCW, are obtained. Comparing with the best known algorithms, we show that our two-step routing approach yields noticeably lower call blocking probability, shorter active-path length, and additional flexibility of adjusting backup-path length (depending on the SWP variant adopted). Besides, our two-step routing approach gives a much shorter running time than the ILP approach, which makes it more suitable for dynamic routing.     

Routing bandwidth guaranteed paths with local restoration in label switched networks

The emerging multiprotocol label switching (MPLS) networks enable network service providers to route bandwidth guaranteed paths between customer sites. This basic label switched path (LSP) routing is often enhanced using restoration routing which sets up alternate LSPs to guarantee uninterrupted connectivity in case network links or nodes along primary path fail. We address the problem of distributed routing of restoration paths, which can be defined as follows: given a request for a bandwidth guaranteed LSP between two nodes, find a primary LSP, and a set of backup LSPs that protect the links along the primary LSP. A routing algorithm that computes these paths must optimize the restoration latency and the amount of bandwidth used. We introduce the concept of "backtracking" to bound the restoration latency. We consider three different cases characterized by a parameter called backtracking distance D: 1) no backtracking (D=0); 2) limited backtracking (D=k); and 3) unlimited backtracking (D=/spl infin/). We use a link cost model that captures bandwidth sharing among links using various types of aggregate link-state information. We first show that joint optimization of primary and backup paths is NP-hard in all cases. We then consider algorithms that compute primary and backup paths in two separate steps. Using link cost metrics that capture bandwidth sharing, we devise heuristics for each case. Our simulation study shows that these algorithms offer a way to tradeoff bandwidth to meet a range of restoration latency requirements.     

WDM网基于混合共享和SRLG约束的通路保护

研究了网状波分复用（WDM）网中动态生存性路由配备问题，提出了一种新颖的基于共享风险链路组（SRLG）束的混合共享通路保护（MSPP）方案。MSPP为每个业务请求分配丁作通路和SRLG分离的保护通路，因此能完全保护单SRLG故障。与传统的共享通路保护（SPP）方案不同，在满足某些约束条件下，MSPP允许部分工作通路和保护通路共享资源。仿真结果表明，MSPP性能优于SPP。     

Multi‐constrained Shortest Disjoint Paths for Reliable QoS Routing

Finding link‐disjoint or node‐disjoint paths under multiple constraints is an effective way to improve network QoS ability, reliability, and so on. However, existing algorithms for such scheme cannot ensure a feasible solution for arbitrary networks. We propose design principles of an algorithm to fill this gap, which we arrive at by analyzing the properties of optimal solutions for the multi‐constrained link‐disjoint path pair problem. Based on this, we propose the link‐disjoint optimal multi‐constrained paths algorithm (LIDOMPA), to find the shortest link‐disjoint path pair for any network. Three concepts, namely, the candidate optimal solution, the contractive constraint vector, and structure‐aware non‐dominance, are introduced to reduce its search space without loss of exactness. Extensive simulations show that LIDOMPA outperforms existing schemes and achieves acceptable complexity. Moreover, LIDOMPA is extended to the node‐disjoint optimal multi‐constrained paths algorithm (NODOMPA) for the multi‐constrained node‐disjoint path pair problem.     

A new approach for fast segment‐based protection in WDM mesh networks

We propose a new approach for developing segment‐based schemes for protection against single link/node failure in wavelength division multiplexing (WDM) mesh networks. In the proposed approach, every request is allocated a pair of link disjoint but most coupled primary and backup paths. Two paths are said to be most coupled if they share the maximum number of end nodes of some existing requests. Coupled paths reduce the total number of hops need to be traversed by a failure signal and, hence, potentially reduces the overall recovery time. We show that the problem of finding a pair of disjoint and most coupled paths is NP‐complete. Accordingly, we propose an efficient and fast protection algorithm called SPXP—Segment Pre‐Cross‐Connected Protection, to allocate disjoint and most coupled paths. The proposed SPXP algorithm reduces the recovery time by ensuring that backup resources are pre‐configured along each backup segment and, hence, is readily available upon a failure. Simulation results for different incremental traffic models and network topologies show that, for most cases, the proposed SPXP exhibits better performance in terms of blocking probability, resource usage, and recovery time compared with existing protection schemes. Copyright © 2010 John Wiley & Sons, Ltd.     

On the improvement of network resource utilization efficiency for the establishment of static dependable connections in SRLG-constrained WDM translucent networks

In this paper, we investigate the problem of establishing static connections with fault-tolerant requirements, also known as dependable connections, taking into account quality of transmission constraints. To the best of our knowledge, this is the first study that tackles the aforementioned problem under shared risk link group (SRLG) constraints in translucent WDM optical mesh networks where typically a set of strategically localized network nodes are equipped with regeneration capability to overcome physical-layer impairment effects. A novel cross-layer heuristic approach is introduced to solve the problem for an heterogeneous networked scenario relying on a cost-effective two-stage protection procedure which combines the well-known path protection and partial path protection schemes in order to ensure instantaneous recovery from any SRLG-failure event. The proposed heuristic integrates a generic auxiliary graph model that incorporates various network heterogeneity factors such as the number of transceivers at each network node, the number of wavelengths on each fiber link, and the regeneration capability of each node, represented by different edges in the constructed graph. Moreover, the integrated auxiliary graph can be applied efficiently to model either single- or mixed-line-rate translucent WDM optical networks wherein different modulation formats are employed in order to support the transmission at different line rates. Our solution approach aims at maximizing the total number of accommodated requests by reducing network resource consumption through the simultaneous use of the backup–backup and primary–backup multiplexing techniques. We, here, present extended versions of these two techniques that generalize the sharing concept to some other important node resources—specifically, regeneration equipments which constitute the major cost factor in optical transport networks—in addition to link resources (i.e., wavelength channels). As far as we know, this is the first attempt to deploy simultaneously generalized versions of the backup–backup and primary–backup multiplexing techniques when considering static traffic patterns without compromising the 100 % fault-recoverability guarantee. The performances of the proposed heuristic are evaluated and discussed through extensive numerical experiments carried out on different network topologies. Significant improvements are demonstrated, either in terms of network blocking performance or in terms of resource utilization efficiency, in comparison with previously proposed approaches.     

Maximally node and SRLG-disjoint path pair of min-sum cost in GMPLS networks: a lexicographic approach

Path protection is a fast and capacity-efficient approach for increasing the availability of end-to-end connections. However, sometimes it is not possible to obtain a fully disjoint path pair. In this case, it may be admissible to consider a path pair which is as disjoint as possible, and thus provide the best (in a certain sense) level of the single-fault protection that can be ensured using this type of approach. A shared risk link group (SRLG) is a group of links which have a common risk of failure. Two new heuristics for solving the min-sum maximally node and SRLG-disjoint path pair are presented. The relative performance of the new heuristics and also of two other previously proposed heuristics is evaluated using four different networks. Results, regarding accuracy and execution time of the studied heuristics, show that one of the new proposed algorithms can be a good compromise for use in the Generalized Multi-protocol Label Switching control plane.     

网状WDM网中多播业务的共享保护设计

研究网状波分复用(WDM)光网络中动态多播业务的保护方案,提出一种共享保护和重配置(SPR)算法.该算法根据网络状态动态调整链路代价,为每个多播业务请求建立最小代价工作树,并为光树上互不重叠的工作段提供链路分离的保护段.当网络中发生链路失效时,进行业务段保护切换和局部资源重配置.仿真表明,该算法可以合理共享波长资源、平衡网络负载,有效保护WDM网中任意单链路失效,并在多链路失效情况下大大提高业务恢复能力.     

Approximating optimal spare capacity allocation by successive survivable routing

The design of survivable mesh based communication networks has received considerable attention in recent years. One task is to route backup paths and allocate spare capacity in the network to guarantee seamless communications services survivable to a set of failure scenarios. This is a complex multi-constraint optimization problem, called the spare capacity allocation (SCA) problem. This paper unravels the SCA problem structure using a matrix-based model, and develops a fast and efficient approximation algorithm, termed successive survivable routing (SSR). First, per-flow spare capacity sharing is captured by a spare provision matrix (SPM) method. The SPM matrix has a dimension the number of failure scenarios by the number of links. It is used by each demand to route the backup path and share spare capacity with other backup paths. Next, based on a special link metric calculated from SPM, SSR iteratively routes/updates backup paths in order to minimize the cost of total spare capacity. A backup path can be further updated as long as it is not carrying any traffic. Furthermore, the SPM method and SSR algorithm are generalized from protecting all single link failures to any arbitrary link failures such as those generated by Shared Risk Link Groups or all single node failures. Numerical results comparing several SCA algorithms show that SSR has the best trade-off between solution optimality and computation speed.     

Dual-Link Failure Resiliency Through Backup Link Mutual Exclusion

Networks employ link protection to achieve fast recovery from link failures. While the first link failure can be protected using link protection, there are several alternatives for protecting against the second failure. This paper formally classifies the approaches to dual-link failure resiliency. One of the strategies to recover from dual-link failures is to employ link protection for the two failed links independently, which requires that two links may not use each other in their backup paths if they may fail simultaneously. Such a requirement is referred to as backup link mutual exclusion (BLME) constraint and the problem of identifying a backup path for every link that satisfies the above requirement is referred to as the BLME problem. This paper develops the necessary theory to establish the sufficient conditions for existence of a solution to the BLME problem. Solution methodologies for the BLME problem is developed using two approaches by: 1) formulating the backup path selection as an integer linear program; 2)developing a polynomial time heuristic based on minimum cost path routing. The ILP formulation and heuristic are applied to six networks and their performance is compared with approaches that assume precise knowledge of dual-link failure. It is observed that a solution exists for all of the six networks considered. The heuristic approach is shown to obtain feasible solutions that are resilient to most dual-link failures, although the backup path lengths may be significantly higher than optimal. In addition, the paper illustrates the significance of the knowledge of failure location by illustrating that network with higher connectivity may require lesser capacity than one with a lower connectivity to recover from dual-link failures.     

A Novel Shared Segment Protection Algorithm for Multicast Sessions in Mesh WDM Networks

This paper investigates the problem of protecting multicast sessions in mesh wavelength‐division multiplexing (WDM) networks against single link failures, for example, a fiber cut in optical networks. First, we study the two characteristics of multicast sessions in mesh WDM networks with sparse light splitter configuration. Traditionally, a multicast tree does not contain any circles, and the first characteristic is that a multicast tree has better performance if it contains some circles. Note that a multicast tree has several branches. If a path is added between the leave nodes on different branches, the segment between them on the multicast tree is protected. Based the two characteristics, the survivable multicast sessions routing problem is formulated into an Integer Linear Programming (ILP). Then, a heuristic algorithm, named the adaptive shared segment protection (ASSP) algorithm, is proposed for multicast sessions. The ASSP algorithm need not previously identify the segments for a multicast tree. The segments are determined during the algorithm process. Comparisons are made between the ASSP and two other reported schemes, link disjoint trees (LDT) and shared disjoint paths (SDP), in terms of blocking probability and resource cost on CERNET and USNET topologies. Simulations show that the ASSP algorithm has better performance than other existing schemes.     

Resource efficiency improved approach for shared path protection in EONs

For improving the resource efficiency of dynamic shared path protection in elastic optical networks, a survivable RSA (SRSA)-based heuristic algorithm is proposed in the paper. In SRSA, an adaptive adjustment link cost function is devised to effectively select working and protection paths. The cost function sufficiently considers available spectrum resources and the length of light paths for both working and protection paths. In order to achieve high resource efficiency, a spectrum allocation strategy named minimal cost stable set is proposed to allocate spectrum for protection paths with respect to the resource efficiency in the link cost function. And the graph coloring algorithm is introduced to select the shared protection path with the highest resource efficiency for the request. Compared with the shared path protection and dynamic load balancing shared path protection, simulation results show that the proposed SRSA decreases bandwidth blocking probability and achieves high resource efficiency.     

Capacity Efficiency and Restorability of Path Protection and Rerouting in WDM Networks Subject to Dual Failures

Resilient optical networks are predominately designed to protect against single failures of fiber links. But in larger networks, operators also see dual failures. As the capacity was planned for single failures, disconnections can occur by dual failures even if enough topological connectivity is provided. In our approach the design of the network minimizes the average loss caused by dual failures, while single failures are still fully survived. High dual failure restorability is the primary aim, capacity is optimized in a second step. For WDM networks with full wavelength conversion, we formulate mixed integer linear programming models for dedicated path protection, shared (backup) path protection, and path rerouting with and without stub-release. For larger problem instances in path rerouting, we propose two heuristics. Computational results indicate that the connectivity is of much more importance for high restorability values than the overall protection capacity. Shared protection has similar restorability levels as dedicated protection while the capacity is comparable to rerouting. Rerouting surpasses the protection mechanisms in restorability and comes close to 100% dual failure survivability. Compared to single failure planning, both shared path protection and rerouting need significantly more capacity in dual failure planning.     

A Dynamic Backup Path Management Method for TDMA Based WiMAX Client WMNs

This paper proposes a backup path management method for time division multiple access (TDMA) based client wireless mesh networks (WMNs). In a TDMA based client WMN, as links/nodes fail or as nodes perform handover and as flows enter and leave the network, the paths between various nodes change as well as the bandwidth available along these paths. In these networks, to support the quality of service requirements of flows, backup paths with the required bandwidth need to be established dynamically. Some methods are proposed in the literature to establish backup paths which handle link/node failures and node handover in ad hoc networks, but none of these methods can provide backup paths with the required bandwidth dynamically. To address that issue, the present paper proposes a backup path management method which is adaptive to both topological changes and traffic changes in a network. Each node along the current path between a source and a destination finds backup paths with the required bandwidth in order to handle failure of the link to its downstream node and its own failure or handover. Nodes use two-hop neighborhood information and slots status information of two-hop neighbors to establish backup paths. We prove that the number of backup paths available when a node N searches for backup paths to handle its own failure are more than the number of backup paths available when some other node searches for the backup paths for the failure of node N. Performance of the proposed method is compared with the performance of a naive path management (NPM) method in which always the source establishes backup paths whenever a link/node fails or a node performs handover, and also with the performance of a backup path management method proposed in the literature. The proposed method significantly outperforms the NPM method and the method selected from the literature. For example, when the speed of the mobile nodes is 50 m/s, the packet delivery ratio with the proposed method is 63 % more than the NPM method and 35 % more than the method selected from the literature.     

Failure protection in layered networks with shared risk link groups

The shared risk link group (SRLG) has been widely recognized as a fundamental concept in layered network design by the industry. However, several issues related to SRLG protections that are of both theoretical interest and practical importance have not been explored fully. Two major issues are avoiding failures caused by "traps" in finding backup paths, and minimizing the total network capacity requested by active and backup paths. In this article, we highlight the significance of the trap problem in layered networks with SRLG and evaluate the performance of several existing SRLG protection schemes in terms of trap avoidance and bandwidth efficiency, as well as their complexities. We also demonstrate that a simple yet intelligent heuristic algorithm can achieve good performance.