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.     

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 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.     

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.     

A dynamic routing algorithm with load balancing heuristics for restorable connections in WDM networks

Dynamic routing of a restorable connection requires a pair of link-disjoint primary and backup lightpaths to be found online when a connection request arrives at the network. We present a distributed dynamic routing algorithm for restorable connections that uses load balancing heuristics in both primary and backup path computations to achieve low demand blocking. The key idea is to assign costs to links so that heavily loaded links will be avoided in the routing of the primary and backup paths and links with a high chance of including a sharable backup channel will be included in the backup path. Simulation results showed that the algorithm performs significantly better than a simple distributed algorithm and achieves comparable performance as a centralized algorithm.     

Architecture for restorable call allocation and fast VP restorationin mesh ATM networks

This paper presents an architecture for restorable call allocation and fast virtual path (VP) restoration in mesh ATM networks. In this architecture, virtual working and spare capacities needed for call allocation and restoration are reserved and released dynamically on a call-by-call basis at the time of call admission and termination. This obviates the need for advance assignment of spare and working capacities. To shorten the call processing delay, this is done in a parallel-distributed fashion. To provide restorable call allocation, parallel-distributed call processing algorithms of sender-chooser type are suggested. The algorithms integrate, on the call level, virtual bandwidth allocation, virtual spare-capacity assignment, and fixed, alternate, or state-dependent routing. Each routing scheme leads to a particular tradeoff between call processing complexity, call setup delay, and bandwidth efficiency. For each pair of nodes, two sets of VPs are provisioned. The first, working VP (WVP) set, is used for call allocation during the normal operation. The second, spare VP (SVP) set, is used for WVP restoration in the event of failures of network elements. Each SVP protects a preassigned subset of the node pair's WVPs. Each SVP is selected to be link/node disjoint from the WVPs that it is assigned to protect. This assures a protection of the WVP set by a small number of SVPs. Since SVPs are preset and appropriate virtual spare capacities are reserved in advance, the architecture guarantees full restorability and provides very fast restoration. The restoration is done on the VP level in a self-healing manner. The suggested architecture requires only local information to be maintained at each node     

Efficient distributed restoration path selection for shared mesh restoration

In MPLS/GMPLS networks, a range of restoration schemes will be required to support different tradeoffs between service interruption time and network resource utilization. In light of these tradeoffs, path-based end-to-end shared mesh restoration provides a very attractive solution. However, efficient use of bandwidth for shared mesh restoration strongly relies on the procedure for selecting restoration paths. We propose an efficient restoration path selection algorithm for restorable connections over shared bandwidth in a fully distributed MPLS/GMPLS architecture. We also describe how to extend MPLS/GMPLS signaling protocols to collect the necessary information efficiently. To evaluate the algorithm's performance, we compare it via simulation with two other well-known algorithms on a typical intercity backbone network. The key figure of merit for restoration bandwidth efficiency is restoration overbuild, i.e., the extra bandwidth required to meet the network restoration objective as a percentage of the bandwidth of the network with no restoration. Our simulation results show that our algorithm uses significantly less restoration overbuild (63%-68%) compared with the other two algorithms (83%-90%).     

Efficient Distributed Bandwidth Management for MPLS Fast Reroute

As service providers move more applications to their IP/MPLS (multiple protocol label switching ) backbone networks, rapid restoration upon failure becomes more and more crucial. Recently MPLS fast reroute has attracted lots of attention as it was designed to meet the needs of real-time applications, such as voice over IP. MPLS fast reroute achieves rapid restoration by computing and signaling backup label switched path (LSP) tunnels in advance and re-directing traffic as close to failure point as possible. To provide a guarantee of bandwidth protection, extra bandwidth has to be reserved on backup paths. Using path merging technique as described in IETF RFC 4090 only, the network is able to share some bandwidth on common links among backup paths of the same service LSP, i.e., so-called intra-sharing. But no solution is provided on how to share bandwidth among backup paths of different service LSPs, i.e., so-called inter-sharing. In this paper, we provide an efficient distributed bandwidth management solution. This solution allows bandwidth sharing among backup paths of the same and different service LSPs, i.e., both intra-sharing and inter-sharing, with a guarantee of bandwidth protection for any single node/link failure. We also propose an efficient algorithm for backup path selection with the associated signaling extensions for additional information distribution and collection. To evaluate our schemes, we compare them via simulation with the basic MPLS fast reroute proposal, IETF RFC 4090, on two networks. Our simulation results show that using our bandwidth management scheme can significantly reduce restoration overbuild from about 250% to about 100%, and our optimized backup path selection can further reduce restoration overbuild to about 60%.     

Restorable dynamic quality of service routing

The focus of quality-of-service routing has been on the routing of a single path satisfying specified QoS constraints. Upon failure of a node or link on the path, a new path satisfying the constraints has to be established. However, resources needed to satisfy the QoS requirements are not guaranteed to be available at the rerouting instant, so QoS is not guaranteed upon failure. Restorable QoS routing, where active and backup paths must be simultaneously set up, has been previously studied. This is mostly motivated by the incorporation of mechanisms to establish QoS guaranteed paths with failure protection in multiprotocol label switching networks. This article describes some previously developed algorithms for dynamic routing of restorable QoS guaranteed paths     

Providing quality-of-protection classes through control-message scheduling in DWDM mesh networks with capacity sharing

This paper considers the problem of providing quality-of-protection (QoP) classes and improving the failure-recovery time performance in dense wavelength-division-multiplexing (DWDM) networks that use mesh-based path restoration schemes with capacity sharing. We focus on the signaling process required for reconfiguring the nodes along the preplanned restoration paths, and propose a novel approach for reducing the restoration time and meeting the QoP requirements by coordinating the setup procedures for the backup paths through scheduling. We present priority-based online scheduling algorithms that are amenable to distributed implementation for the problems of: 1) minimizing the worst case restoration time and 2) maximizing the number of connections that meet their QoP-class-specific restoration time deadlines. We also present mixed-integer-linear-program (MILP) formulations for both problems for comparison purposes. The online scheduling methods that we propose use simple connection and/or class-specific information and can be easily implemented with minor modifications to the currently proposed signaling protocols. We apply these methods to signaling protocols that require cross-connect configurations at different nodes to be done in sequence as in the current generalized multiprotocol label switching specification, as well as signaling protocols that allow cross-connect configurations to be done in parallel. It is shown that in both cases, significant performance improvements are achievable through scheduling in terms of both the QoP grades that can be supported and the restoration times, with both the MILP solutions and the heuristics. The improvement in restoration time and restorability through our heuristics can be quite high (e.g., increase from a network restorability performance of 40% to a network restorability of 88%, and a 12% reduction in worst case restoration time).     

Algorithms for computing QoS paths with restoration

There is a growing interest among service providers to offer new services with Quality of Service (QoS) guarantees that are also resilient to failures. Supporting QoS connections requires the existence of a routing mechanism, that computes the QoS paths, i.e., paths that satisfy QoS constraints (e.g., delay or bandwidth). Resilience to failures, on the other hand, is achieved by providing, for each primary QoS path, a set of alternative QoS paths used upon a failure of either a link or a node. The above objectives, coupled with the need to minimize the global use of network resources, imply that the cost of both the primary path and the restoration topology should be a major consideration of the routing process. We undertake a comprehensive study of problems related to finding suitable restoration topologies for QoS paths. We consider both bottleneck QoS constraints, such as bandwidth, and additive QoS constraints, such as delay and jitter. This is the first study to provide a rigorous solution, with proven guarantees, to the combined problem of computing QoS paths with restoration. It turns out that the widely used approach of disjoint primary and restoration paths is not an optimal strategy. Hence, the proposed algorithms construct a restoration topology , i.e., a set of bridges, each bridge protecting a portion of the primary QoS path. This approach guarantees to find a restoration topology with low cost when one exists.     

Ad hoc网络中不相关多路由源端寻路算法

路由方案是Ad hoc网络中一个热点研究领域。其中,按需路由算法由于其有效性在带宽受限的Ad hoc网络中得到比较大的发展。然而大部分的按需路由算法,建立并只使用单条路由,当前使用的路径的链路断开时,路由算法必须执行一个路由修复过程。提出了不相关多路由源端路由算法（DMSR）,建立并利用多条最大不相关路由。算法中,中间节点等待一段时间以得到多个路由请求包（RREQ）,然后在这个RREQ中,选择相关性最小的多路径,并将这些信息写入一个RREQ中,并将它广播出去。从仿真结果可以看出本文的算法提高了数据包的正确传输率和业务均衡性。     

Spare Capacity Allocation in Two-Layer Networks

In this paper we consider the problem of provisioning spare capacity in two-layer backbone networks using shared backup path protection. First, two spare capacity allocation (SCA) optimization problems are formulated as integer linear programming (ILP) models for the cases of protection at the top layer against failures at the bottom layer. The first model captures failure propagation using overlay information between two layers for backup paths to meet diversity requirements. The second model improves bandwidth efficiency by moving spare capacity sharing from the top layer to the bottom layer. This exposes a tradeoff between bandwidth efficiency and extra cross-layer operation. Next, the SCA model for common pool protection is developed to allow spare capacity sharing between two layers. Our previous SCA heuristic technique, successive survivable routing (SSR) is extended for these optimization problems. Numerical results for a variety of networks indicate that the common pool protection is attractive to enhance bandwidth efficiency without loss of survivability and that the SSR heuristic quickly results in near optimal solutions     

Cost Effective Shared Path Protection for WDM Optical Mesh Networks with Partial Wavelength Conversion

In this paper, we study routing and wavelength assignment of connection requests in survivable WDM optical mesh networks employing shared path protection with partial wavelength conversion while 100% restorability is guaranteed against any single failures. We formulate the problem as a linear integer program under a static traffic model. The objective is to minimize the total cost of wavelength-links and wavelength converters used by working paths and protection paths of all connections. A weight factor is used which is defined as the cost ratio of a wavelength converter and a wavelength-link. Depending on the relative cost of bandwidth and wavelength conversion, the optimization objective allows a proper tradeoff between the two. The proposed algorithm, the shortest-widest-path-first (SWPF) algorithm, uses a modified Dijkstra's algorithm to find a working path and a protection path for each connection request in the wavelength graph transformed from the original network topology. When there are multiple candidate paths that have the same minimum total cost, the path along which the maximum number of converters used at each node is minimized is chosen by the SWPF algorithm. We have evaluated the effectiveness of the proposed algorithm via extensive simulation. The results indicate that the performance of the proposed algorithm is very close to that of the optimal solutions obtained by solving the ILP formulation and outperforms existing heuristic algorithms in terms of total number of converters used and the maximum number of converters required at each node in the network. The proposed algorithm also achieves slightly better performance in terms of total cost of wavelength-links and converters used by all connections. We also investigated shared path protection employing converter sharing. The results show that the technique can reduce not only the total number of converters used in the network but also the maximum number of converters required at each node, especially when a large number of converters are needed in the network. In this study, although the ILP formulation is based on static traffic, the proposed algorithm is also applicable to routing dynamic connection requests.     

Incremental survivable network design against node failure in SDH/SONET mesh networks

Network survivability is becoming more and more important for the plenty of information each single fiber carries. Extra network resources are needed to increase network survivability level. In this paper, we investigate the problem of how to augment the network topology with adding new links and allocate spare capacity to maximize the service restorability against node failures in SDH/SONET mesh networks. A scheme called maximal node-disjoint backup paths provisioning with topology augmentation is proposed to tackle the problem, and another scheme called globally optimized path provisioning with topology augmentation, which allows adjusting the existing working paths of network flows, is investigated to optimize the augmented network globally. Both schemes are formulated as mixed integer linear programming models. Furthermore, heuristic algorithms are investigated to be implemented in software. Three algorithms, i.e., added links searching method, successive maximal survivable routing method, and random sequence routing convergence method, are designed and compared. Simulation results show the effectiveness of the algorithms.     

Optimal capacity placement for path restoration in STM or ATMmesh-survivable networks

The total transmission capacity required by a transport network to satisfy demand and protect it from failures contributes significantly to its cost, especially in long-haul networks. Previously, the spare capacity of a network with a given set of working span sizes has been optimized to facilitate span restoration. Path restorable networks can, however, be even more efficient by defining the restoration problem from an end to end rerouting viewpoint. We provide a method for capacity optimization of path restorable networks which is applicable to both synchronous transfer mode (STM) and asynchronous transfer mode (ATM) virtual path (VP)-based restoration. Lower bounds on spare capacity requirements in span and path restorable networks are first compared, followed by an integer program formulation based on flow constraints which solves the spare and/or working capacity placement problem in either span or path restorable networks. The benefits of path and span restoration, and of jointly optimizing working path routing and spare capacity placement, are then analyzed     

Extending the p-cycle concept to path segment protection for span and node failure recovery

The paper introduces an extension to the method of p-cycles for network protection. The p-cycle concept is generalized to protect path segments of contiguous working flow, not only spans that lie on the cycle or directly straddle the p-cycle. The original span protecting use of the p-cycle technique is extend to include path protection or protection of any flow segment along a path. It also gives an inherent means of protecting working flows that transit a failed node. We use integer linear programming to study the new concept and determine its inherent capacity requirements relative to prior p-cycle designs and other types of efficient mesh-survivable networks. Results show that path-segment-protecting p-cycles ("flow p-cycles") have capacity efficiency near that of the shared backup path-protection (SBPP) scheme currently favored for optical networking. Because its protection paths are fully preconnected and because it protects path segments (not entire paths), it has the potential for both higher speed and higher availability than SBPP. We also develop capacity optimization models to support 100% restoration of transiting flows through failed nodes. Only a very small additional spare capacity is needed to achieve both 100% span and intermediate node-failure restorabilities, and a very high transiting traffic restorability can be accomplished for node failure restorability given spare capacity only for span-failure protection. An immediate practical application is to suggest the use of flow p-cycles to protect transparent optical express flows through a regional network.     

Simple Pre-Provisioning Scheme to Enable Fast Restoration

Supporting fast restoration for general mesh topologies with minimal network over-build is a technically challenging problem. Traditionally, ring-based SONET networks have offered close to 50 ms restoration at the cost of requiring 100% over-build. Recently, fast (local) reroute has gained momentum in the context of MPLS networks. Fast reroute, when combined with pre-provisioning of protection capacities and bypass tunnels, enables faster restoration times in mesh networks. Pre-provisioning has the additional advantage of greatly simplifying network routing and signaling. Thus, even for protected connections, online routing can now be oblivious to the offered protection, and may only involve single shortest path computations. In this paper, we are interested in the problem of reserving the least amount of the network capacity for protection, while guaranteeing fast (local) reroute-based restoration for all the supported connections. We show that the problem is NP-complete, and we present efficient approximation algorithms for the problem. The solution output by our algorithms is guaranteed to use at most twice the protection capacity, compared to any optimal solution. These guarantees are provided even when the protection is for multiple link failures. In addition, the total amount of protection capacity reserved by these algorithms is just a small fraction of the amount reserved by existing ring-based schemes (e.g., SONET), especially on dense networks. The presented algorithms are computationally efficient, and can even be implemented on the network elements. Our simulation, on some standard core networks, show that our algorithms work well in practice as well     

On surviving dual-link failures in path protected optical WDM mesh networks

In this paper, we investigate the problem of enhancing dual-failure restorability in path protected mesh-restorable optical Wavelength Division Multiplexed (WDM) networks. Recent studies have demonstrated the need to survive simultaneous dual-link failures and have also provided solutions for handling such failures. A key finding of these early efforts is that designs providing complete (i.e. 100%) protection from all dual-failures need almost triple the spare capacity compared to a system that protects against all single-link failures. However, it has also been shown that systems designed for 100% single-link failure protection can provide reasonable protection from dual-link failures [M. Clouqueur, W. Grover, Mesh-restorable networks with 74 enhanced dual-failure restorability properties, in: Proc. SPIE OPTICOMM, Boston, MA, 2002, pp. 1-12]. Thus, the motivation for this work is to develop a hybrid mechanism that provides maximum (close to 100%) dual-failure restorability with minimum additional spare capacity.The system architecture considered is circuit-switched with dynamic arrival of sessions requests. We propose an adaptive mechanism, which we term active protection, that builds upon a proactive path protection model (that provides complete single-failure restorability), and adds dynamic segment-based restoration to combat dual-link failures. The objective is to optimize network survivability to dual-link failures while minimizing additional spare capacity needs. We also propose a heuristic constraint-based routing algorithm, which we term best-fit, that aids backup multiplexing among additional spare paths towards this goal. Our findings indicate that the proposed active protection scheme achieves close to complete (100%) dual-failure restorability with only a maximum of 3% wavelength-links needing two backups, even at high loads. Moreover, at moderate to high loads, our scheme attains close to 16% improvement over the base model that provides complete single-failure restorability. Also, the best-fit routing algorithm is found to significantly assist backup multiplexing, with around 15%-20% improvement over first-fit at all loads. The segment-based restoration algorithm reiterates the importance of utilizing wavelength converters in protection and is seen to provide around 15%-20% improvement over link restoration especially at moderate to high loads.     

A novel shared-path protection algorithm with correlated risk against multiple failures in flexible bandwidth optical networks

In this paper, to decrease the traffic loss caused by multiple link failures, we consider the correlated risk among different connection requests when both the primary and backup paths are routed and assigned spectrum. Therefore, a novel shared-path protection algorithm is developed, named shared-path protection algorithm with correlated risk (SPP_CR), in flexible bandwidth optical networks. Based on the correlated risk, the routing can be diverse and the sharing in backup spectral resource will be restricted by SPP_CR algorithm, then the dropped traffic caused by simultaneous multiple failures between primary and backup path can be efficiently decreased. Simulation results show that, SPP_CR algorithm (i) achieves the higher successful service ratio (SSR) than traditional shared-path protection (SPP), shared-path protection with dynamic load balancing (SPP_DLB) and dedicated path protection (DPP); (ii) makes a better tradeoff in blocking probability, protection ratio (PR), average frequency slots consumed (AFSC) and redundancy ratio (RR) than SPP, SPP_DLB and DPP algorithms.