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134 JOURNAL OF NETWORKS, VOL. 5, NO. 2, FEBRUARY 2010 switching nodes there may be multiple data links and it is therefore more efficient to manage these as a bundle using a single separated out-of-band control channel. In recent years, the notion of an optical control plane has received extensive attention and has rapidly developed to a detailed set of protocol standards, currently being standardized by the International Telecom munication Union-Telecommunication Standardization Sector (ITU-T) and others [5]. Nevertheless, several additional issues in terms of security and network management are still unsolved [3]. One of the main management issues revolves around the fact that optical performance monitoring techniques for AONs are not a well developed and mature technology. As a result, performance and impairment information can not be used in order to ensure better QoS. For example, when discussing routing in AONs, it is usually assumed that all routes have adequate signal quality (ensured by limiting AONs to sub-networks of limited size). This approach is very practical and has been applied to date when determining the maximum length of optical links and spans, for example. Specifically, operational considerations such as failure isolation also make limiting the size of domains of transparency very attractive. Another example concerns the Routing and Wavelength assignment (RWA) problem, where network blocking has traditionally been estimated using analytical and simulation approaches without taking transmission impairments into consideration [4]. Efficient connection provisioning, however, requires more than simply advertising the availability of wavelengths and routes to switching nodes. Hence, RWA approaches require additional control information to be taken into consideration including the characteristics and performance measurements of established lightpaths in the network. Thus, the combination of both RWA and performance information may offer the prospect of improvements in the provisioning of lightpaths in AONs [6]. However, dealing with this challenge necessitates improving available RWA mechanisms to update and Protection Network survivability Fault survivability Attack survivability Detection Localization Identification advertise the performance measurements needed for assigning efficient routes and pre-computing their restoration paths. IV. NETWORK SURVIVABILITY A crucial feature of any communication network is its survivability which refers to the ability to withstand component failures and to continue providing services in disruption conditions. Providing resilience against failures is therefore an important requirement for many high-speed networks. As these networks carry more and more data, the amount of disruption caused by a network fault or attack becomes more and more significant. Protection switching is the key technique used to ensure network survivability. Protected connections require a secondary path so that when a fault degenerates enough the primary path, the information can be transmitted through the secondary path (which can be reserved or used by pre-emptive traffic depending on the protection scheme). Another used technique requiring less network resources and redundant capacity is restoration. In this approach, the secondary path is computed when the fault occurs and therefore, although it may be slower than protection, it adapts better to the available capacity that the network has in that moment. Another important advantage of restoration is that it can handle multiple simultaneous failures; whereas protection techniques are designed for handling a preset number of failures; typically single fiber failures. Both techniques are usually implemented in a distributed manner to ensure faster restoration of services after the occurrence of single (or multiple) fault(s). Since many data-centric services may not require hard guarantees on the recovery time, restoration techniques are more suitable than protection techniques. From a management viewpoint, network survivability involves primary the protection of secure data. As shown in Fig. 1, network survivability can be broadly divided in two categories, namely, fault survivability and attack Physical security Management Reaction Isolation Restoration Semantic security Management Authentication Authorization Cryptographic control Figure 1. Survivability in optical networks. Fault and attack management is a key management function for ensuring the secure and continues functioning of the network. But this is still a major complication in the development of secure AONs. © 2010 ACADEMY PUBLISHER
JOURNAL OF NETWORKS, VOL. 5, NO. 2, FEBRUARY 2010 135 survivability. The main goals of fault survivability are to provision lightpaths in anticipation of faults, locate the faults, and to restore the affected connections. Attack survivability can be, in turn, subdivided into two separate but still interrelated types: Physical security and semantic security. The former promises to ensure integrity and privacy of information, as well as the QoS by protecting the network against service disruption and service degradation. The latter focuses on the protection of information even when an attacker has access to the transmission data channel. Fault and attack management is one of the crucial functions and a prerequisite for the above mentioned protection and restoration schemes. In this regard, there are many complications that prevent the continuous control and monitoring of all supported lightpaths simultaneously. An important implication of using AON components in communication systems is that available methods used to manage and monitor the health of the network may no longer be appropriate. Therefore, without additional control mechanisms, an attack upon the core network might not be detectable. In an AON, a lightpath will be capable of carrying analogue and digital signals with arbitrary bit rates and protocol formats. Such a lightpath typically traverses multiple data links and there are many components along its route that can fail. Since a transmission fiber carries a number of highbandwidth channels, its failure will cause simultaneous multiple channel failures, resulting in large amounts of data being corrupted or compromised. Since AON components will not by design be able to comprehend signal modulation and coding, intermediate switching nodes are unable to regenerate data, making segment-bysegment testing of communication links more demanding. As a direct consequence, failure detection and localization using existing integrity test methods is made particularly difficult. In recent years, several studies related with the management of faults and attacks in AONs have been reported [7-12]. Worthy of mentioning is a series of management methods proposed in [3, 7]. In particular, the authors have presented an extensive analysis of AON vulnerabilities to security attacks. As countermeasures, they have proposed new methods for detecting, localizing, and identifying of attacks that may be practiced upon AONs. Layer n-1 Layer n Layer n+1 Manager Agent (a) Manager EMS Another approach [2, 8] proposes an algorithm that solves the multiple failure location problem in transparent optical networks where the failures are more deleterious and affect longer distances. The proposed solution also covers the non-ideal scenario, where lost and/or false alarms may exist. Although the problem of locating multiple faults has been shown to be NP-complete, even in the ideal scenario where no lost or false alarms exist, the proposed algorithm keeps most of its complexity in a pre-computational phase. Hence, the algorithm only deals with traversing a binary tree when alarms are issued. This algorithm locates the failures based on received alarms and the failure propagation properties, which differ with the type of failure and the kind of network equipment used in the network. Another algorithm has been proposed in [1, 9] to correlate multiple security failures locally at any node and to discover their tracks through the network. The algorithm is distributed and relies on a reliable management system since its overall success depends upon correct message passing and processing at the local nodes. To identify the source and nature of detected performance degradation, the algorithm requires up-todate connection and monitoring information of any established lightpath, on the input and output side of each node in the network. This algorithm mainly runs a generic localization procedure, which will be initiated at the downstream node that first detects serious performance degradation at an arbitrary lightpath on its output side. Once the origins of the detected failures have been localized, the NMS can then make accurate decisions (for example, which offender lightpaths should be disconnected or rerouted) to achieve finer grained recovery switching actions. However, most of these works are still in progress and need to be embedded and tested within standardized control planes. V. MANAGEMENT FRAMEWORK Given the complexity of network management and diversity requirements, a valid question to ask is whether a centralized NMS would be better than decentralized management methods. Although most of available management systems are implemented in a centralized manner, this manner of implementation is rather slow and inadequate for optical networks because of the large Element Management Layer Agent Agent Agent Agent Lightpath NMS Detection points EMS Network Management Layer Pi Pi+2 Pj Pj+3 Pk Pk+1 Figure 2. Management and functional architecture in a typical optical network. In this functional model each agent reports to its associated EMS which in turn reports to the domain or global management system. © 2010 ACADEMY PUBLISHER (b)
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Aims and Scope. Call for Papers and
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