Abstract— A key element of the survivability cost is the amount of the additional capacity embedded in the network for recovery purposes. Generally, Internet Service Providers (ISPs) have the obvious aim of achieving the required level of survivability with minimum resource consumption and network cost. Therefore, in order to achieve such a challenge, it is necessary to move towards the multilayer differentiated survivability concept.

In this paper, the focus is given to the investigation of the application of differentiated survivability concept with pre-allocated restoration technique considering a distributed GMPLS-based IP-over-optical mesh network under single and dual-link failure scenarios.

Index Terms— Multi-layer survivability, Quality of Recovery, pre-allocated restoration, GMPLS-based networks, differentiated survivability concept.

I. INTRODUCTION

The two layer structure, comprising an the IP layer built directly over optical layer, has been identified as a promising candidate for the structure of future telecommunication networks. Moreover, Generalised multi-protocol label switching (GMPLS) has been developed by the Internet Engineering Task Force (IETF) to form an intelligent control plane for such a structure. This control plane is able to establish and tear down lightpaths on-demand and to provide quality of service (QoS) [1-2]. One of the critical issues in this structure is network survivability.

Survivability mechanisms can be implemented in the GMPLS-based IP-over-optical network using three techniques; protection, restoration and pre-allocated restoration techniques [3]. The pre-allocated restoration scheme, considered in this work, bridges the gap between the protection and restoration techniques. It is very simple in terms of implementation and operation. In this scheme, additional capacity is embedded in the network specifically for survivability purposes. This additional capacity is not visible to the routing algorithms under normal operation. Moreover, a pre-allocated restoration technique is more flexible in terms of resource utilization and coping with various failure scenarios, whereby routing computation and resource allocation need only be involved once a failure has occurred.

The pre-allocated restoration technique can be provided at either one or both layers. Consequently, for both economic and operational aspects, efficient and flexible multilayer survivability mechanisms are absolutely necessary [4]. The economic issue is related to the amount of required physical spare capacity and an ability to provide

quality of recovery (QoR) for customers. On the other hand, the operational issues are related to the efficiency of multilayer survivability mechanisms in resolving different failure scenarios. In practice, the design of a multilayer survivability schemes requires the provision of cooperation mechanisms between control planes in different layers. Such cooperation must consider many important issues such as resource utilization (sharing of spare capacity), scalability (routing information and implementation) and providing the required QoS.

From QoS perspective, the dominant parameters used to classified services are delay, packet loss and jitter delay [5]. However, it is expected that the GMPLS-based IP-over-optical network runs a variety of applications with different survivability requirement. Therefore, in such a structure, it is essential to include the survivability parameters within the class of service specification [6]. Such parameters include recovery time and restorability.

Most of the previous studies attempted to consider the differentiated survivability based on single layer where the class of services placed on either the IP or optical layer [7]. Some recent studies [8-9] attempted to provide multilayer differentiated survivability. The key challenge is to provide an efficient and practical mapping framework between different classes placed on different layer. Such studies provide a mapping framework based on protection and restoration techniques.

Beside the protection, it is clearly demonstrated that the pre-allocated restoration technique can be considered as an alternative technique to provide guaranteed protected connections [3]. Consequently, this paper investigates the application of differentiated survivability concept with pre-allocated restoration technique, in particular, the multilayer class of service mapping considering a distributed GMPLS-based IP-over-optical mesh network.

The paper is organized as follows; Section II describes the differentiated survivability in the pre-allocated restoration technique. Model implementation and system assumptions are considered in Section III. Section IV presents model performance and simulation results. Finally the paper is concluded in Section V.

II. DIFFERENTIATED SURVIVABILITY CONCEPT IN PRE-ALLOCATED RESTORATION TECHNIQUE

Considering the two-layer structure, the class of services can be provided at the optical, IP, or both layers. At the IP layer, the classes are represented logically with no physical resources dedicated for each class. At the optical layer, the

Differentiated Survivability in a Distributed GMPLS-Based IP-over-Optical Network

David Harle and Saud Albarrak University of Strathclyde, Glasgow, UK { d.harle,Sbarrak@eee.strath.ac.uk }

`classes of services are characterised physically whereby each class routes over a certain topology as illustrated in Figure 1. The reason is that the optical layer cannot offer a sub wavelength level of service since its granularity is an entire wavelength.

As can be seen from this Figure, from the survivability perspective, it is possible to route several IP classes within one optical class topology under the condition that these classes require the same level of survivability. Such an approach is practical since the number of classes at the optical layer is limited and is affected directly by the available resource.

In order to implement the multilayer differentiated survivability in the path-level pre-allocated restoration technique, two significant issues need to be considered; providing quality of recovery (QoR) and multilayer class of service mapping function.

Providing quality of recovery (QoR) with path-level pre-allocated restoration is possible using a recovery class prioritisation method proposed in a previous work [3]. Such a method is implemented based on the restoration time where discrete intervals of time are pre-defined and each class is associated with one of these intervals. For instance, class one can tolerate t1 delay time, class 2 can tolerate t2 delay time and so on. It is possible to apply the retrial method within any time-interval for enhanced performance in the corresponding class. Therefore, when the recovery process fails, the restoration process is repeated. It is observable that the first class of service will achieve the best performance in terms of restoration time and restoration ratio followed by the second class and so on. From the implementation perspective, the class prioritisation method is an efficient method in terms of scalability (the only information required is the delay time associated with each class of service), simplicity (operates independently at each node). Moreover, it is simple to implement under the GMPLS protocols.

The multilayer class of service mapping function is realised by means of survivable grooming policies embedded in the edge router admission control. Such policies are implemented based on a number of constraints associated with traffic engineering strategies and survivability techniques. The main concerns with such implementation are flexibility and scalability. Based on the pre-allocated restoration technique, the survivable traffic grooming does not involve survivable routing computation under normal operation. Therefore, in order to achieve the mapping function, only partial information is required. This information describes the level of survivability provided by optical classes on one hand, and the level of survivability required for each IP class on the other. Such information is generally preset and does not involve on-line updating. Consequently, based on the pre-allocated restoration, it is clear that the mapping policy is scalable in terms of required information and implementation.

III. MODEL IMPLEMENTATION

This work uses the OMNeT++ (Objective Modular Network Testbed in C++) discrete-event simulation platform which supports modelling of distributed mesh topologies. The network structure consists of a set of nodes connected by a set of paired fibre links. Internally, each node consists of an edge router connected to an optical cross connection (OXC) as shown in Figure 2. The network structure can be seen from two perspectives: a data plane and a control plane. The data plane must be an overlay model with three topologies: link, lightpath, and Label Switched Path (LSP) topology. The control planes, in both the edge routers and OXCs, consist of three units: the signalling, the routing, and the recovery unit. The functionalities of the signalling and routing units are implemented using standard GMPLS protocols [10]. The node units require particular information in order to efficiently implement their functionality. Specifically, several data tables are maintained in each node. These tables can be updated by either signalling or routing protocols. The signalling protocol facilities table updating that maintain local information such as wavelength routing, lightpath information, forwarding and LSP information tables. On the other hand, tables that maintain global information including the link resource availability and logical topology tables are updated by means of the routing protocol. This model considers three delay components; the link propagation delay, the link transmission delay and the nodal process delay.

At the IP layer, LSP connections are requested and terminated randomly with requests arriving based on a Poisson process. The LSP parameters include the source, the destination, class of service and the capacity selected randomly based upon a uniform distribution. From the routing calculation perspective, this model adopts the source explicit routing concept and the constraint-based shortest-path-first algorithm. The former considers the provision of an explicit route at the source nodes, therefore, this route cannot be modified during the signalling phase. The latter algorithm provides an efficient method to compute the route of connections. The IP routing unit determines the explicit

IP layer: N classes

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Figure 1: An example of class of services interaction in the IP-over-optical networks

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Figure 3: The blocking probability versus the number of classes at the IP and optical layers respectively.

(a): Different classes at IP layer

(b): Different classes at both the optical and IP layer

route based on the amount of available capacity in each lightpath while the optical layer determines the explicit route based on the number of free wavelengths in each link. Routers request a new lightpath base on the lightpath-create-first policy [11]. Based on such a policy, edge routers search for a direct lightpath within the existing lightpath topology. If there is no lightpath available, the edge router requests a new lightpath from its associated OXC to accommodate the new LSP requests. At each lightpath setup failure, the routers attempt to find a route within the existing lightpaths. Therefore, an LSP could traverse multiple lightpaths between source and destination. The request will be blocked if there are no available resources along its route. It is assumed that no repeat behaviour is considered under normal condtions. Lightpaths are terminated if they are not being traversed by LSPs.

Failures are generated randomly. The inter-arrival time and holding time of failures are generated based on an exponential distribution. Links selected for failures are obtained using a uniform distribution. The dual link failure scenario considered in this work is when two random links fail simultaneously. The path-level recovery (end-to-end recovery) is applied which provides better resource utilisation than link-level. The recovery procedure can be classified in to three key processes: fault notification, failed connection teardown, recovery.

•Notification process: The notification process starts at the upstream node, which is responsible to send a notify message to the source node. It is possible to aggregate many failed connections that belong to the same source node on one notify message. The optical layer is responsible for notifying the IP layer about any unrecovered or unprotected lightpath.

•Teardown process: The teardown process starts at both the upstream and downstream node. The upstream node is responsible for tearing down the upstream segment while

the downstream node is in charge of the teardown of the downstream segment.

•Recovery process: The recovery process starts at the source node of any failed connection. The recovery process requires the provisioning of an alternative connection. Therefore, the admission control, at the source node, meets this challenge by using existing available capacity including the pre-allocated spare capacity. The pre-allocated spare capacity is reserved Using ‘a link partitioning’ method at optical layer and the ‘lightpath partitioning’ method at IP layer [3].

IV. PERFORMANCE RESULTS

A. The Impact of Class of Service Placement This section investigates the impact of assigning several

classes at the optical and IP layers on the model performance in terms of blocking probability. In this experiment it is assumed that all values are recorded at the same network load (400 Erlangs) where the network topology adopted in this work is the NSFnet network topology. Figure 3a illustrates the blocking probability of the model against the number of classes assigned in IP with the assumption that there are no classes provided by the optical layer. Figure 3b demonstrates the blocking probability versus the number of the class provided by the optical layer assuming that the IP layer provides three classes of service. The experimental results show that the blocking probability significantly increases when the

Figure 2: IP-over-optical data and control plane structure

Packets from higher layers

Packets classification

LSPsswitching

Router Optical Local Port

OXC Local Port

OXCPortSwitching

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number of classes at the optical layer is increased while there is no significant impact for the number of classes at the IP layer on the blocking probability. This result is expected whereby the class of services at the optical layer is represented physically by assigning a lightpath topology for each class.

B. The single-layer Class Prioritisation Method performance

This experiment investigates the class prioritisation method performance at either optical or IP layer and its impact on the required spare capacity, in particular, with a dual-link failure scenario. It is assumed that all values are recorded at the same network load (400 Erlangs) with two retrials. The percentage of classes routed in the network are 20% class EF (Expedited Forwarding), 30% class AF (Assured Forwarding), and 50% class BE (Best Effort). The differentiation between classes pertains to the same categories used in differentiated services (DiffServ) traffic with each class having a different priority. Two scenarios are investigated:

- Scenario 1: the spare capacity is allocated at the IP layer based on the lightpath partition method. The three classes (EF, AF and BE) are assigned by IP layer while the optical layer provides only the un-recovered lightpath class. Therefore, IP is fully responsible for the recovery process.

- Scenario 2: the spare capacity is allocated at the optical layer based on the link partition method. The three classes (EF, AF and BE) are assigned at the IP layer. The

optical layer provides two classes: recovered and un-recovered lightpath class. The former is recovered at the optical layer or partially at the IP layer, while the latter is recovered partially at the IP layer. It is assumed that class EF is routed over the recovered lightpath topology, while the other two classes (AF and BE) are routed over the un-recovered lightpath topology.

Figure 4 illustrates the restoration ratio for the three classes in the two scenarios respectively. The particular interest in both scenarios is the EF class. The experimental results show that the performance of class EF is improved when the reserved capacity is increased in both scenarios. Additionally, it achieves the best performance when compared to the other classes. Moreover, Figure 5.7b shows that the restoration ratios of both AF and BE classes do not improve, even though the number of reserved wavelengths is increased. The reason being that, based on the second scenario, the two classes attempt to recover at the IP layer, while the spare capacity is embedded at the optical layer. Furthermore, Figure 4 clearly demonstrates the importance of applying differentiated survivability, in particular with dual link failures. For example, in order to support full dual link failures of class EF, the required spare capacity is 2 Gb/s spare capacity (20% of lightpath capacity) based on scenario one and 2 wavelengths (25% of link capacity) in the second scenario. the results show clearly that, in order to achieve full dual link failure recovery without using QoR, the amount of spare capacity embedded in the network should be in the range of 40-50% of the total network capacity.

C. Multilayer Class Prioritisation Performance The main intention of this experiment is to investigate

providing multilayer quality of recovery (QoR) based on the pre-allocated restoration technique. From the survivability perspective, the characteristic of the DiffServ-like classes is described in table one.

Table 1: the recovery requirements of the DiffServ-like classes

Class Restorability Recovery time Recovery layer

EF Guaranteed recovery Lowest Optical, using link

partitioning method

AF Guaranteed recovery Medium IP, using lightpath

partitioning method BE Best effort highest IP

In this experiment, based on the suggested framework, the spare capacity is allocated at the both layer. It is assumed that the reserved spare capacity is fixed (2 wavelengths at each link and 2Gb/s of any active lightpath). The optical layer provides two classes: recovered and un-recovered lightpath class. The former is recovered at the optical layer while the latter is recovered partially at the IP layer. It is assumed that class EF is routed over the recovered lightpath topology, while the other two classes (AF and BE) are routed over the un-recovered lightpath topology. This experiment considers a dual-link failure scenario with two retrials.

Figure 5a and 5b illustrate the restoration ratio and

Figure 4: The single layer class prioritisation method performance.

(a): the first scenario performance

(b): the second scenario performance

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restoration time for the three classes against the network offered load. The experimental results show that both EF and AF classes achieve guaranteed recovery Additionally, the recovery time of class EF is the lowest in comparison with class AF and BE. The experiments show that the suggested framework of multilayer differentiated survivability is an efficient and is simple to implement under the GMPLS protocols.

V. CONCLUSION

Focus in this paper was given to the multilayer pre-allocated restoration performance, in particular the differentiated survivability concept. The performance was obtained based on a simulation model built using OMNET++ considering a distributed GMPLS-based IP/WDM network model. Two issues were investigated. Firstly, the impact of class of service placement was investigated. Secondly, the single- and multi-layer class prioritisation method applied to the differentiated survivability concept was described. Additionally, the model performance was investigated under dual-physical link failure scenarios.

The experimental results show that the blocking probability significantly increases when the number of classes at the optical layer is increased while there is no significant impact for the number of classes at the IP layer. Moreover, the results clearly demonstrate the importance of

applying differentiated survivability, in particular with dual link failures, to redued the required spare capacity. A multilayer differentiated survivability framework was proposed based on pre-allocated restoration technique. The results show that such a framework is an efficient mechanism to provide the QoR and easy to implement under the GMPLS protocols.

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[5] Mario Marchese, Qos over Heterogeneous Networks, John Wiley & Sons, 2007.

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[7] Wayne D. Grover, Mesh-Based Survivable Networks, Prentice Hall PTR, 2004.

[8] Qin Zheng, and Mohan G., Protection approaches for dynamic traffic in IP/MPLS-over-WDM networks, in IEEE Communications Magazine, Vol.41, No.5, May 2003.

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Figure 5: The multi-layer class prioritisation method performance.