Smita Rai

IP resilience within an autonomous system: current approaches, challenges, and future directions

Network survivability is gaining increasing attention from the Internet research community. The ubiquity of IP services has fueled increasing interest in ensuring their dependability, by making IP networks more disruption-tolerant. After providing a brief overview of how routing is accomplished in the Internet, this article reviews how the protocols react to failures or changes in network state within an autonomous system. The problems associated with current restoration schemes, with respect to newer and more stringent requirements posed by emerging services such as voice over IP, are identified. We present an overview of the schemes proposed to ameliorate fault recovery and critique their contributions. We also outline future research directions for improving IP resilience.

Reliable multipath provisioning for high-capacity backbone mesh networks

We investigate reliable multipath provisioning of traffic in high-capacity backbone mesh networks, e.g., next-generation SONET/SDH networks supporting virtual concatenation (VCAT). VCAT enables a connection to be inversely multiplexed on to multiple paths, a feature that may lead to significantly improved performance over conventional single-path provisioning. Other mesh networks such as those employing optical wavelength-division multiplexing (WDM) and multiprotocol label switching (MPLS) can also benefit from this multipath provisioning approach. We propose effective multipath bandwidth as the metric to provision a connection while satisfying its reliability requirements (measured in terms of availability). We demonstrate that effective multipath bandwidth provides more flexibility and lower blocking probability without the cost and the complexity associated with traditional protection schemes developed for optical WDM and MPLS networks. We also investigate the practical problem of provisioning effective multipath bandwidth with cost constraints. We analyze the tractability of the problem and present a heuristic which results in significantly reduced number of blocked connections due to cost constraints.

Extension of segment protection for bandwidth efficiency and differentiated quality of protection in optical/MPLS networks

This paper investigates the problem of dynamic survivable lightpath provisioning against single-node/link failures in optical mesh networks employing wavelength-division multiplexing (WDM). We unify various forms of segment protection into generalized segment protection (GSP). In GSP, the working path of a lightpath is divided into multiple overlapping working segments, each of which is protected by a node-/link-disjoint backup segment. We design an efficient heuristic which, upon the arrival of a lightpath request, dynamically divides a judiciously selected working path into multiple overlapping working segments and computes a backup segment for each working segment while accommodating backup sharing. Compared to the widely considered shared-path protection scheme, GSP achieves much lower blocking probability and shorter protection-switching time for a small sacrifice in control and management overhead. On the basis of generalized segment protection, we present a new approach to provisioning lightpath requests according to their differentiated quality-of-protection (QoP) requirements. We focus on one of the most important QoP parameters-namely, protection-switching time-since lightpath requests may have differentiated protection-switching-time requirements. For example, lightpaths carrying voice traffic may require 50 ms protection-switching time while lightpaths carrying data traffic may have a wide range of protection-switching-time requirements. Numerical results show that our approach achieves significant performance gain which leads to a remarkable reduction in blocking probability. While our focus is on the optical WDM network, the basic ideas of our approaches can be applied to multi-protocol label switching (MPLS) networks with appropriate adjustments, e.g., differentiated bandwidth granularities.

Survivable Differential Delay Aware Multi-Service Over SONET/SDH Networks with Virtual Concatenation

We propose a model for differential-delay compensation and two approaches for provisioning survivable service on Data-over-SONET/SDH networks with virtual concatenation. Our approaches optimize the resource subject to the constraint of differential-delay compensation.

On provisioning in all-optical networks: an impairment-aware approach

We investigate connection provisioning in an all-optical wavelength-division multiplexing (WDM) network in the presence of physical-layer impairments. As the channel bit rate increases, impairments pose a more serious problem, and lightpaths need to be routed intelligently, so that the destination node receives the signal with adequate quality. We study the models of major physical impairments that affect optical signals. With reasonable assumptions, we model the major impairments as link-based metrics and we formulate schemes for routing, which consider the impairment constraints imposed by the underlying physical infrastructure. Prior work in this area has focused on enumerating a set of paths based on criteria such as minimum hops/distances, and then selecting a path from this set based on complex calculations of signal quality. Our approach integrates verification of impairment constraints while searching for paths. Routing with multiple additive constraints being NP-hard, we give an optimal heuristic algorithm with fast execution time for practical networks, as well as a dynamic programming pseudo-polynomial-time algorithm for the case when one impairment constraint dominates. We also examine the case of a practical ¿all-optical¿ network with signal regeneration facility at some nodes. We explore routing in such a translucent network and discover that finding a simple feasible path passing through a regenerator is NP-complete.

Provisioning in Ultra-Long-Haul Optical Networks

We present a new approach to provision connections in ultra-long-haul optical networks with physical impairments. Our cross-layer approach finds paths satisfying impairment constraints while taking into account regenerators (3R) present in the network.

New Approach to Reliable Multipath Provisioning

We study the problem of reliably provisioning traffic in high-capacity backbone mesh networks supporting virtual concatenation (VCAT). Traditional approaches handled reliability requirements using full protection, which offers high assurance but can be costly. We take a less expensive approach by offering an expected bandwidth, rather than an absolute amount, but at lower costs than full-protection approaches. We propose a new routing algorithm that uses minimum-cost flow to find efficient collections of paths that satisfy traffic requests for expected bandwidth. We find that in most realistic network settings, paths are reliable enough so that only few additional network resources are required to compensate for the bandwidth loss incurred from possible network failures. We compared our approach to a previous algorithm under both a uniform setting, with symmetric traffic distribution and equal edge capacities, and in a more realistic setting with asymmetric traffic and differing edge capacities. Results show that our algorithm is an attractive approach in both settings, and much more effective than the previous scheme. We also compare our approach to a full-protection approach. Results show that by using fewer network resources, our algorithm is able to satisfy significantly more traffic requests than full protection. Thus, when full protection is not crucial, using our routing approach should be beneficial.

A novel approach to provision differentiated services in survivable IP-over-WDM networks

IP-over-WDM networks are starting to replace legacy telecommunications infrastructure and they form a promising solution for next-generation networks (NGNs). Survivability of an IP-over-WDM network is gaining increasing interest from both the Internet research community and service providers (SPs). We consider a novel static bandwidth-provisioning algorithm to support differentiated services in a survivable IP-over-WDM network. We propose and investigate the characteristics of both integer linear program (ILP) and heuristic approaches to solve this problem. In the heuristic method, we propose backup reprovisioning to ensure network resilience against single-node or multiple-link failures. Illustrative examples compare and evaluate the performance of the two methods in terms of capacity-usage efficiency and computation time.

On Provisioning in Dual-Node Interconnected SONET/SDH Rings

SONET/SDH is historically the dominant telecom transport infrastructure for backbone networks, and it is optimized for reliable delivery of voice and private-line services. Efficient utilization of the existing infrastructure through novel methods and algorithms can lead to higher revenue, which is attractive for network operators. In this study, we examine a network of inter-connected SONET/SDH rings that use dual-node interconnection employing the drop-and-continue facility, which is the de-facto standard for interworking SONET/SDH protection architectures. We develop an efficient algorithm for provisioning in such a network while keeping fragmentation of capacity low and taking into account the various constraints imposed by the underlying physical layer. We examine the case of dual-node-interconnected ring networks employing both contiguous and virtual concatenation (a next-generation SONET/SDH architecture), and as expected, we discover that provisioning higher-bandwidth connections with virtual concatenation offers significant improvement. The stringent time-slot alignment and contiguity constraints imposed by contiguous concatenation are further compounded by the constraints imposed by the ring interworking architecture, and virtual concatenation allows capacity to be used more efficiently.

A Better Approach to Reliable Multi-Path Provisioning

We study the problem of reliably provisioning traffic in high-capacity backbone mesh networks supporting virtual concatenation (VCAT). VCAT enables a connection to be inversely multiplexed on to multiple paths, a feature that has many advantages over conventional single-path provisioning. We propose improved routing algorithms which use minimum- cost flow to find efficient collections of paths that satisfy the traffic requests. We first investigate the performance of our scheme under a uniform setting with symmetric traffic distribution and equal link capacities. We then apply our algorithm in a more realistic setting with asymmetric traffic and differing link capacities. Our algorithm is effective in both the uniform and non-uniform settings, and is much more effective than previously proposed schemes. Our study in the non-uniform setting is significant as it gives insight into the performance of our algorithm under more realistic scenarios.