A Disjoint Path Selection Scheme Based on Enhanced ... - IEEE Xplore

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE GLOBECOM 2005 proceedings.

A Disjoint Path Selection Scheme Based on Enhanced Shared Risk Link Group Management for Multi-reliability Service Takashi Miyamura† Takashi Kurimoto Akira Misawa Shigeo Urushidani NTT Network Service Systems Laboratories, NTT Corporation 3-9-11 Midori-cho, Musashino-shi, Tokyo, 180-8585 Japan Tel: +81-422-59-3527 Fax: +81-422-59-3494 E-mail: † [email protected] Abstract— We consider a mechanism for providing a multi-reliability service in multilayer GMPLS networks. By introducing GMPLS restoration techniques and shared risk link group management, we can provide a highly reliable protected connection service. However, there is a trade-off between reliability and efficiency of network resource usage. In addition, reliability requirements differ depending on the type of service (e.g., Internet access or leased line). Thus, we propose a mechanism for calculating an efficient route for a protected connection that can satisfy specific reliability conditions requested by customers. We also present simulation results that indicate our schemes are remarkably effective for achieving a better balance between end-toend reliability and efficiency. We also demonstrate the quantitative relationship between availability of a service and resource efficiency through simulation experiments.

I. Introduction Broadband Internet access services such as ADSL (asymmetric digital subscriber line) and FTTH (fiber to the home) have been very popular recently. The traffic volume imposed on core networks has been rapidly increasing because of the increasing number of broadband subscribers as well as advances in broadband access technologies. One of the major concerns is constructing a backbone network that offers the economical transport of large-scale IP packet traffic while achieving high reliability. Next generation Internet backbone network/node architectures have been extensively studied [1], [2], [3], [4]. To achieve the economical transport of high-capacity IP traffic, a backbone network should be based on optical technologies such as dense wavelength division multiplex (DWDM) transmission systems and optical cross-connect (OXC) systems. Optical-layer transport technologies can scale well in terms of bandwidth capacity, but their bandwidth granularity is coarse and allocated only to a fixed unit of wavelength bandwidth such as 2.5 Gbps and 10 Gbps. On the other hand, packet-by-packet processing cannot support high-capacity bandwidth and can be more expensive in some regions. However, packet-by-packet processing offers finer granularity of bandwidth as well as perflow QoS guarantees, unlike the optical layer path. At each transit node, several low-speed packet paths can be merged into one packet or optical path. To achieve better utilization of network resources, packet multiplexing needs to be supported. Moreover, considering expenditure with regard to network operation, that is OPEX (operational expenditure), a backbone network has basically consisted of multiple layers such as IP, SONET/SDH, and optical layers for serveral years. Each layer has been operated and managed independently by different administrative domains within a service provider organization. This architecture complicates network operation because of the lack of coordination between two different layers, which leads to additional OPEX. Tighter coordination between multiple layers is required to overcome these issues in current networks. Our research project has been motivated by the above observation. Our research group has proposed the next generation backbone architecture, called a Multilayer Service Network (MLSN) architecture, as illustrated in Fig. 1, that provides multilayer IP and optical network services [1], [2] with an interlayer coordination function. Requirements for the next generation backbone network include i) economical transport of large-capacity IP traffic, ii) multi-QoS traffic support, iii) providing multiple reliability reliability, and iv) incorporating multiple different layer service networks.

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Fig. 1. NTT’s next generation backbone network architecture Multi-layer service network (MLSN) architecture. MLSN consists of IP and optical layers with interlayer coordination technologies and accommodates heterogeneous multiple layer service networks on a single infrastructure. PCE (path computation element) provides path computation function and also controls multilayer path topology to re-optimize network resource usage according to traffic demand variation

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One of the biggest challenges for developing such backbone networks is to provide different reliability conditions required by each service network on a single shared network infrastructure. The backbone network needs to provide various service conditions required by each service, as shown in Fig. 2. Mechanisms for providing multi-QoS have been extensively studied [3], while few papers have examined network design for providing multiple reliability conditions. Hence, our goal is to develop a mechanism for providing a multi-reliability service on a shared backbone network. For reliability, LSP (label switched path) protection is a commonly used approach to enhance the availability of a network service. Many research groups have proposed LSP protection mechanisms, which use multiple disjoint LSPs between pairs of nodes [5], [6], [7], [8], [9], [10], [11]. Recently, from the viewpoint of reliability, algorithms for finding a pair of SRLG (shared risk link group) [13]-disjoint paths have received much attention. Without considering SRLG, we cannot ensure the 100% recovery performance of path protection, even for a single point of failure in the network. Qiao et al. [10] proposed an SRLG-disjoint routing algorithm, called PROMISE, which provides SRLG-failure-independent protection by introducing a multi-segment protection technique. While a protection mechanism that takes SRLG-disjointness into account provides high availability, it consumes far more network resources than conventional node/link-failureindependent protection mechanisms. Basically, high reliability is traded-off for resource efficiency. Thus, for the economical

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transport of various types of services, it is essential to provide a path satisfying a certain degree of reliability required by each service while minimizing resource consumption. We consider a mechanism for achieving a multi-reliability service in a single backbone network consisting of multiple layers. We propose a multi-reliability path selection algorithm that provides a certain level of availability while achieving an efficient use of network resources. The basic process of the proposed algorithm is to calculate the route for a pair of primary and backup paths that minimize the sharing of SRLG by considering the reliability of physical resources. The key to the mechanism lies in enhanced shared risk group management that enables us to identify physical network structures and the reliability of each resource in the network. The rest of the paper is organized as follows. In Section 2, we briefly review the MLSN concept and then present the multi-reliability service followed by some related work. Next, we propose a method for finding a pair of paths for the multireliability service in Section 3. Then, we report on the results of the extensive simulations we performed to evaluate the effectiveness and feasibility of our algorithm in Section 4. A brief conclusion is provided in Section 5. II. Background In this section, we briefly introduce the MLSN concept and a multilayer GMPLS (generalized multi-protocol label switching) [14] network, then define the multi-reliability service, which is offered to service networks by a multilayer GMPLS network. We also address some related work with regard to path protection mechanisms and shared risk group management, and point out that existing technologies are not sufficient to achieve the multi-reliability service that we consider in this paper. A. MLSN Concept and Multilayer GMPLS Network We have proposed the MLSN concept as a suitable basis for a next generation converged backbone network [1], [2]. Here, we briefly review the MLSN concept and demonstrate its superiority over existing network architecture. Requirements to create a next-generation backbone network with heterogeneous multiple layers to serve a multiple different layer network will be as follows. a) Economical transport of high-capacity IP traffic and multiQoS traffic support. b) High reliability and stability. c) Incorporate multiple different layer service networks. Though a GMPLS network based on the “peer model” can satisfy requirements a) and b), it cannot incorporate multiple IP-based networks of different users. This is because the peer model cannot provide independence between the control plane of a backbone network and those of multiple service networks. MLSN architecture solves this problem. The MLSN extends GMPLS to serve multiple IP networks and enhances operability for carriers and nationwide ISPs. An MLSN consists of MLS edge nodes, MLS core nodes and PCE (path computation element) servers. In addition, it offers multi-layer connection service to service networks. An MLS core is a simple OXC-based system that supports GMPLS protocols. An MLS edge is an IP-router-based system that provides edge functionalities including IP packet processing, multiplexing of low-speed packet traffic, and tunneling of control messages among service networks belonging to the same user. It supports GMPLS protocols for path control within the backbone network and legacy IP-based protocols such as OSPF (open shortest path first) and BGP (border gateway protocol) for interworking with IP service networks. The main features of the MLS edge are a horizontal interworking function. The control plane of the MLS network and IP service networks have independent and virtually separated addressing spaces for different service networks to achieve interoperation seamlessly. Service network control messages, like OSPF and RSVP messages, can be tunneled through the MLSN. To provide the tunnelling function, some address conversion functions may be needed at the MLS edge to resolve the address contention

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between multiple service networks. Such functionality is called as “horizontal interworking”. A PCE is an entity that is capable of computing a network path or route based on different policies of service networks in a multi-layer service network. PCEs can be located on an MLS edge, or on an out-of-network server. A PCE collects resource information within the core network using a routing protocol like OSPF-TE [15] or a protocol specific to traffic engineering like GTEP [16]. Hereafter, we only discuss network design and resource control within the core network, so our proposal can be easily applicable to conventional network architecture other than the MLSN architecture. We call a GMPLS network consisting of optical and packet layers a multilayer GMPLS network. B. Multi-reliability Service Here, we introduce a multi-reliability service in a multilayer GMPLS network. To design a protected connection with high reliability, it is highly important to avoid simultaneous failures on both primary and backup paths. The SRLG concept has been developed in designing failure recovery mechanisms [13]. If a primary LSP and its backup LSP are assigned to different wavelengths in the same fiber, a single fiber cut would result in the failure of both primary and backup LSPs. To avoid this problem, the SRLG concept has been introduced to ensure the disjointness of those two LSPs. The concept of multi-reliability service was originally proposed by Kawamura and Ohta [11], [12]. In [11], reliability is measured by the restoration probability, which is defined as the number of successfully restored paths normalized by the number of paths affected by underlying failure. However, their measure is considered to be inappropriate from the view point of customers because it does not consider the frequency of failure occurrence. Thus, customers cannot estimate service interuption time within a certain time span, that is, unavailability. Note that availability/unavailability is widely used for designing customers applications on a carrier’s connectivity service. For example, customers usually request 99.999% of availability from a service provider or less than five-minute downtime in a year, not 95% of restoration probability. In addition, their approach did not consider the physical structure of a network (i.e., SRLG) only a single logical layer structure. Thus, their approach cannot avoid simultaneous failure because of a single failure of a physical resource (e.g. fiber cut). Note that availability of a connection served by protection paths is mainly determined by frequency of simultaneous failures on both primary and backup paths. From the above observation, we define a multi-reliability service as providing a protected connection service that achieves specific availability requested by a customer with minimum resource usage. In computing the route for a path, we take into account physical structures and resources as well as the logical structure with respect to the C (control)-plane to provide a protected connection service satisfying different reliability requirements. C. Related Work Here, we briefly review related work with regard to disjoint path selection algorithms and clarify why we need to extend existing shared risk group management and develop a new algorithm for providing the multi-reliability service. Saito et al. [7] proposed multipath algorithms, which can be used for load balancing as well as failure recovery in MPLS networks. Their schemes use multiple multipoint-to-point LSPs. Recently, Li et al. [8] proposed a disjoint routing algorithm in a MPLS network. Their algorithm finds a pair of efficient disjoint paths by sharing resources among backup paths. However, those algorithms do not take into account the SRLG (shared risk link group) concept. Recently, from the viewpoint of reliability, algorithms for finding a pair of SRLGdisjoint paths have received much attention. Oki et al. [9] proposed a heuristic SRLG-disjoint path finding algorithm for shared LSP protection in GMPLS networks. The disadvantage of their algorithm is that they cannot find SRLG-disjoint paths

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path selection scheme, called an MRPS (multi-reliability path selection) algorithm, which enables us to provide a protected connection service that can provide a certain degree of reliability requested by each customer while achieving efficient use of network resources. Moreover, we discuss PCE-based network architecture in conjunction with modification of the proposal in [17] in consideration of conceivable problems with regard to network operation.

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efficiently if the network contains “traps” [10]. To avoid the “traps” problem, Qiao et al. [10] proposed an SRLG-disjoint routing algorithm. Their algorithm can avoid “traps” by using a multi-segment protection technique called PROMISE, and provide SRLG-failure-independent protection. While a protection mechanism that takes SRLG-disjointness into account provides high availability, it consumes far more network resources than conventional node/link-failureindependent protection mechanisms. Basically, high reliability is traded-off for resource efficiency. The objective of approaches in [9], [10] is to provide a pair of paths that do not share any physical resources from source to destination. In addition, they assumed that physical networks consist of a single type of physical resources, that is fiber. However, physical networks consist of various types of resources, such as fiber, conduits, WDM systems, and amplifications. The point here is that different types of physical resources exhibit different reliability. To achieve a certain degree of reliability, we do not necessarily consider the disjointness of resources that are less liable to fail compared to other kinds of resources. To mitigate disjoint conditions with regard to some types of resources, we can find an efficient route for a pair of protection paths while satisfying reliability requirements. As described above, the current SRLG specification is used to identify a set of links sharing common physical resources, that is, sharing a risk of the occurence of simultaneous failure. From the SRLG information, we cannot assess availability of a protected connection because of the lack of information about the reliability of physical resources composed of the connection. Recently, Papadimitriou et al. [17] proposed some extensions to SRLG to carry information about physical network structures (e.g. fiber topology) and geographic location of a physical resource identified by the SRLG. To perform path computation while considering availability, a PCE needs to know information about physical resource types that constitute an underlying connection and reliability of each physical resource. It is possible to propagate such information within the backbone network by deploying Papadimitriou’s approach. However, their concern was mainly about protocol design for identifying the relationship between logical network topology and physical network structures, so an algorithm for finding a pair of disjoint paths with different reliability requirements was beyond their scope. Thus, we consider network architecture suitable for multi-reliability service and propose a disjoint

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Here, we present PCE-based enhanced shared risk group architecture for multi-reliability service and then give an overview of the proposed scheme. To perform path selection considering end-to-end availability, a PCE requires information about physical resources composed of protected connections and their reliability. For example, in Fig. 3a, two links are configured between nodes A and B in view of the C-Plane, but these links possess different reliability, which cannot be recognized from SRLG information distributed by conventional OSPF-TE. If we select the link in the Dplane with a dotted line for a connection, reliability of the connection would be degraded compared to the case where the link with a solid line is selected. Consequently, the PCE cannot perform appropriate path selection in consideration of availability by just using link cost available in the current OSPF-TE specification. Moreover, we point out that physical resource information is essential for selecting the efficient route for a path that offers moderate availability. A primary path (the solid line) and two backup paths (the dotted lines) are configured in the network as illustrated in Fig. 3b. Each pair of primary and backup paths shares SRLG on the route. Thus, in this case, a pair of SRLGdisjoint paths cannot be found, unless the strict SRLG-disjoint condition is rectified. However, two SRLG are assigned to two different types of physical resources (a conduit for the upper dotted line and a WDM system for the lower dotted line) on the data plane. If the requested connection requires intermediate reliability (not extreme high reliability like that of the leasedline class), the connection is accepted to allocate the upper dotted line for its backup path because of a low risk of sharing the conduit, which leads to network efficiency improvement. To accommodate these issues, we introduce enhanced shared risk group management based on Papadimitriou’s approach. A PCE-based architecture for providing multi-reliability service is illustrated in Fig. 4. First, in our architecture, information about a physical resource type is carried by extended SRLG proposed by Papadimitriou, as shown in Fig 5. The Type field is used for identifying physical resource types such as fiber, conduit, and WDM system, while the Identifier field provides the identification of different physical resources. In Papadimitriou’s approach, information about physical resource reliability is also propagated by the routing protocol. However, such a specification complicates protocol operations, because availability information could be updated in the course of daily network operations. Additionally, some operator wishes to use availability as a measure of reliability, but another operator may would like to use MTBF (mean time between failure) for this purpose, as availability of equipment also depends on MTTR (mean time to repair), which is difficult to determine at the time of installation, in addition to MTBF. Thus, in our architecture, SRLG is used to carry information about physical resource types and provides its identification at the PCE, and also at the C-Plane. The PCE maintains the database that stores information about availability corresponding to each physical resource type. Now, we explain basic procedures of the proposed scheme. First, upon receiving customer’s request for a protected connection service, the PCE examines required reliability and then selects types of SRLGs that need to consider disjointness. Next, the PCE executes the mulitireliability path selection (MRPS) algorithm, which is described later, for finding a protected connection route. After obtaining the route, the PCE checks

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L1 by lij . In addition, we define edge node group VEdge , where VEdge ⊂ V . VEdge contains nEdge nodes and Vi ∈ VEdge for 1 ≤ i ≤ nEdge (≤ n). Each fiber link has a cost, and we denote L1 the cost of link lij by cL1 ij . A packet layer network is modeled as a graph GP = (VEdge , LP ), where LP is a set of optical links, which correspond to LSC (lambda switch capable)-LSP in the optical layer. We introduce the SRLG to provide the identification of resource types and manage the relationship between the logical topology and the physical network structure. Here, we denote SRLG of type i by SRLGi. SRLG of type i information associated with L1 link lij is indicated by,  L1 1 if ljk is assigned to SRLGi #l, srlgi (j, k, l) = 0 else,

where, i, j ∈ {1, . . . , n}. For example, assume that conduits are L1 L1 assigned to type 2 SRLG and fiber link lkl and lmn share the common conduit SRLG2#j. Then, we can express as follows.

Change path selection policy with more strict disjoint conditions Route for LSP with reliability constraints is obtained

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srlg2 (k, l, j) = srlg2 (m, n, j) = 1.

Flowchart of proposed scheme.

if the protected connection satisfies the reliability requirement. Finally, if it satisfies the requirement, the PCE sends obtained route information to an edge router terminating the connection. Otherwise, the PCE recomputes the MRPS algorithm by modifying a set of SRLG considered in the routing to improve the reliability. In this way, the route for a pair of protection paths with certain availability is obtained. A flowchart of our scheme is shown in Fig. 6. B. Model description Before presenting the MRPS algorithm, we describe a network model and define the problem we are trying to solve. We need an algorithm for finding a pair of paths that share minimal common SRLGs of specified types. Moreover, if we can select the route among multiple candidates, we wish to select the most efficient route with required reliability considering the risk of sharing an underlying physical resource. However, conventional SRLG-disjoint algorithms [5], [9], [10] assumed only one type of SRLG in finding a pair of disjoint paths. Thus, we propose an algorithm to solve this problem. Now we describe the network model to explain our scheme. A multilayer GMPLS network consists of packet, optical, and physical layer networks. The optical layer network is modeled as a graph GL1 = (V, LL1 ), where V is a set of nodes (optical crossconnects or edge routers) and LL1 is a set of fiber links. We assume that GL1 has n nodes. Each node is denoted by Vi (i = 1, . . . , n) and the link between Vi and Vj is denoted

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Network model

In addition, note that an optical LSP is set up only between two edge nodes Vi and Vj (0 ≤ i, j ≤ nEdge ), and the LSP can be seen as an optical link in the packet layer network. Thus, in the initial state, the packet layer network has no optical link, that P is LP = φ. Optical links lij ∈ LP are added whenever an optical LSP is established in the optical layer network. Additionally, a packet LSP is constructed over the packet layer network topology. We also use the following notation. • BWλ : Bandwidth of one wavelength in fiber links • Nλ : Number of wavelengths per one fiber link • Preq : Requested availability • Plink : Unavailability of a node • Pnode : Unavailability of a link i • Psrlg : Unavailability of SRLGi • α: Parameter used for modifying link cost in computing a backup path • αi : Parameter for modifying link cost for the inclusion of SRLGi i • LSPP P : Route for primary LSC LSP#i (i = 1, 2, . . .) i • LSPBP : Route for backup LSC LSP#i (i = 1, 2, . . .) An example of a network model is shown in Fig. 7. Here, there are three edge routers (E1, E2, and E3), five OXCs, and ten fiber links in the optical layer network, and three LSCLSPs are already configured. In the physical layer network, two types of SRLGs are considered, where SRLG types 1 and 2 correspond to DWDM systems and conduits, respectively. Two LSC-LSPs are configured between edge nodes E1 and E2 in the network, which are link/node disjoint in the optical layer. However, considering the physical layer, two LSPs share SRLG#2.1 corresponding to a conduit. Thus, if strict disjoint conditions are required, a pair of disjoint paths cannot be found in this case. Provided that the sharing of a physical resource

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identified by SRLG2 is allowed, a protected connection from E1 to E2 could be accepted, which improves network resource efficiency while achieving a certain level of reliability. The point here is that we need to assess the risk of sharing some SRLGs for a protected connection by considering the improvement of resource efficiency. Now, we define the problem we are solving. The goal is to find a pair of optical-layer paths for a protected connection service that satisfies an availability requirement requested by a customer while minimizing bandwidth consumption.

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Requested bandwidth: 1 wavelength Number of generated connections: 150 Link cost cL1 ij :10 (uniform in the network) Unavailability of nodes Pnode : 0.005 Unavailability of links Plink : 0.005 1 Unavailability of SRLG1 Psrlg : 0.01 2 Unavailability of SRLG2 Psrlg : 0.001 α1 : 5.0 α2 : 10.0

C. Multi-reliability path selection (MRPS) algorithm Now we describe the MRPS algorithm itself, which finds a pair of paths that are maximally disjoint of multiple types of SRLGs. The MRPS algorithm is expressed as follows. • Step A1: According to requested unavailability Preq , select a protection type (e.g. Unprotected, 1+1 Protection, etc.). Then go to Step A2. • Step A2: Compute the route for a primary path LSPPi P by using the constrained shortest path first (CSPF) algorithm. If the selected protection type requires a backup path, then go to Step B1. Otherwise, go to Step A3. • Step A3: After calculating unavailability of the obtained path, check if the unavailability is less than Preq . If the path satisfies the condition, then go to End. Otherwise, go to Step A4. • Step A4: Modify the current protection type to the one that can provide higher reliability. Then, go to Step A2. • Step B1: According to the reliability requirement Preq , determine the set of SRLG types RBP which take into account their disjointness in computing the route for the backup path. Then, go to Step B2. • Step B2: Next, draw up SRLG list S P P included in LSPPi P . L1 ∈ LL1 , compute link set Lsrlg (⊂ LL1 ) that Then, for ljk contains link sharing SRLG in S P P . Then, go to Step B3. • Step B3: Modify link cost on primary path LSPPi P as follows. P 8 L1 α · cL1 > j∈RBP ∪(k,l)∈S srlg αj · ckl kl + > < L1 ∈ LSP i , if l L1 PP kl P ckl = L1 > j∈RBP αj · ckl > : L1 srlg else if lkl ∈ L , Next, based on the modified link cost, compute the route for the i backup path LSPBP by using CSPF. Then, go to Step A3. • End.

In this way, a pair of paths that provide requested reliability can be found. IV. Performance Evaluation To evaluate the effectiveness and feasibility of our proposal, we performed extensive simulations in terms of i) amount of resource consumption for various reliability requirements and ii) unavailability of each algorithm for varying reliability of a physical resources. In this section, we present some of the results. Here, we use the total number of wavelengths occupied by accepted LSPs as the measure of efficiency. As for reliability, unavailability is used as the performance measure. Please refer to [18] for a detailed method for evaluating unavailability of a path. A. Simulation conditions Before presenting the results, we explain some assumptions and parameters used for our evaluation. In the simulations, we used the 11-node and 25-link COST239 network. Logical topology in the optical layer network is illustrated in Fig. 8, and edge nodes, not shown in the figure, are attached to each of a node. LSC-LSPs are configured between a pair of nodes chosen randomly. We assume that a physical layer network consists of two types of resources, so two types of SRLGs are assigned to a link in the optical layer. SRLG1 and SRLG2 correspond to WDM systems and conduits, respectively. Each node is attached to two or three WDM systems, and 22 SRLGs are assigned to 25 optical-layer links. For SRLG2, 18 SRLGs are randomly assigned to the links in the optical layer network. We basically used the following parameters in our experiments.

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Fig. 8. The 11-node and 25-link COST239 network topology used in our simulations.

We use four types of protection algorithms; i) Unprotected, which provides an unprotected connection, ii) Protected, which uses a pair of node/link-disjoint paths for protection, iii) SRLG1, which also considers disjointness of SRLG1 in addition to Protected, and iv) SRLG1/2, which requires disjointness of SRLG1 and SRLG2. Because Unprotected only uses a primary path, we used it as a benchmark of efficiency. SRLG1/2 requires the most strict disjoint conditions, thus it provides higher reliability compared to other algorithms while it consumes more network resources. To evaluate the performance of each algorithm, we demonstrate the feasibility and applicability of our algorithm. B. Trade-off between reliability and efficiency First, we evaluate the efficiency and reliability of each algorithm while varying the number of wavelengths per fiber from 8 to 64. The number of accepted LSPs normalized by the generated LSPs is shown in Fig. 9. Comparing required wavelengths per fiber to achieve 100% of accepted ratio, Protected required only about 48 wavelengths, while SRLG1/2 consumed about 60 wavelengths. From the results, by mitigating the disjoint condition of some physical resources, we can reduce resource consumption by about 25%. Next, we need to assess the risk of sharing some physical resources. Thus, we investigated the performance of each algorithm in terms of efficiency and reliability. Here, the number of wavelengths per fiber was fixed at 64. The results are summarized in Fig. 10. The solid line and the dotted line indicate unavailability and efficiency in each algorithm. In this graph, efficiency is defined as the total network resources allocated to accepted LSPs in each algorithm normalized by those in Unprotected. From this result, we can see algorithms for a protected connection consume 100-150% more resources compared to those of Unprotected. The reliability, that is unavailability of Unprotected was at about 0.027. On the other hand, Protected achieved unavailability of about 0.0028. Thus, Protected improved reliability by one order of magnitude compared to Unprotected. Next, comparing unavailability SRLG1 of with Protected, SRLG1 achieved about 60% lower unavailability than Protected while SRLG1 consumed only 3% more network resources. Thus, SRLG1 improves reliability with slight increase in resource consumptions. However, comparing reliability of SRLG1/2 with SRLG1, the improvement of unavailability by SRLG1/2 was only about 18%. Because SRLG2 was assigned to conduits that possess relatively low reliability compared to other types of resources, the sharing of SRLG2 slightly affected availability of a protected connection. From these results, we confirm the

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Fig. 9. Comparison of accepted LSP ratio normalized by all generated LSPs in each algorithm

Fig. 11. Comparison of unavailability in each algorithm while varying failure rate of SRLG2

References

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SRLG1/2

Comparison of unavailability and efficiency in each

effectiveness of a path selection algorithm in consideration of reliability of each physical resource type. C. Performance of each algorithm for varying unreliability of SRLG2 Finally, we investigated reliability of each protection algorithm while varying unreliability of a physical resource. In this simulation, we varied unreliability of SRLG2 from 0.01 to 0.00001 with fixed unreliability of SRLG1. The result is shown in Fig. 11. The horizontal line indicates unreliability of SRLG2 normalized by that of SRLG1. Normalized unreliability of 0.01, which indicates that unreliability of SRLG2 is lower by two orders of magnitude than SRLG1, we can see there is no significant difference in reliability between SRLG1 and SRLG1/2. Thus, it is not effective to consider the disjointness of SRLG2 to improve reliability. On the other hand, for the normalized unreliability of SRLG2 from 0.1 to 1, we can see SRLG1/2 effectively improved reliability compared to SRLG1. From the above simulation results, we conclude that physical resources with a lower failure rate by two orders of magnitude need not be taken into account in disjoint routing. This observation allows us to utilize network resources efficiently without degrading reliability of connections. V. Concluding Remarks We considered a mechanism for providing a multi-reliability service in multilayer GMPLS networks and proposed an algorithm for finding an efficient route for a path while satisfying a required reliability condition. We also performed extensive simulations, which show our schemes are remarkably effective for achieving better balance between end-to-end reliability and efficiency. We also demonstrated quantitatively the relationship between availability and resource efficiency through simulation experiments.

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[1] T. Kurimoto, T. Miyamura, M. Aoki, I. Inoue, N. Matsuura, H. Kojima, and S. Urushidani,“A proposal of multi-layer service network architecture,” IEICE Techinical Report, PS2003-6, September 2003 (in Japanese). [2] I. Inoue and T. Takeda,“Proposal of multi layer network architecture and related mechanisms for inclusion into fundamental recommendations,” ITU-T contribution, September 2003. [3] N. Yamanaka, T. Kurimoto, T. Miyamura, and M. Aoki,“MSN Type-X: Next generation Internet backbone switch/router architecture,” Proc. IEEE ICC 2002. [4] S. Okamoto, E. Oki, K. Shimano, A.Sahara, and N. Yamanaka, “Demonstration of the highly reliable HIKARI router network based on a newly developed disjoint path selection scheme,” IEEE Commun. Mag., 52-59 November 2002. [5] T. Miyamura, T. Kurimoto, M. Aoki and A. Misawa,“An Interarea SRLG-disjoint Routing Algorithm for Multi-segment Protection in GMPLS Networks,” Proc. ICBN 2004, April 2004. [6] T. Miyamura, T. Kurimoto, M. Aoki, and S. Urushidani,“A Multi-layer Disjoint Path Selection Algorithm for Highly Reliable Carrier Services,” Proc. Globecom 2004, December 2004. [7] H. Saito, Y. Miyaho, and M. Yoshida, “Traffic engineering using multiple multipoint-to-point LSPs,” Proc. IEEE INFOCOM’00, 894-901 2000. [8] G. Li et al., “Efficient distributed path selection for shared restoration connection,” Proc. of IEEE INFOCOM’2002, 2002. [9] E. Oki et al., ”A Disjoint Path Selection Scheme with SRLG in GMPLS networks,” Proc. of IEEE HPSR’2002, 88-92, May 2002. [10] C. Qiao et al., “A Novel Segment Protection Approach for SRLG Networks,” Proc. of HSN’2003, 2003. [11] T. Yahara, R. Kawamura, and S. Ohta, “Multi-reliability Selfhealing Scheme that Guarantees Minimum Cell Rate,” Proc. DRCN’ 98, 1998. [12] Ryutaro Kawamura and Hiroshi Ohta, “Architectures for ATM Network Survivability and their Field Deployment,” IEEE Commun. Mag., No. 8, pp. 88-94, August 1999. [13] J. Strand and A.L. Chiu, “Issues for routing in the optical layer”, IEEE Commun. Mag., 39, no. 2, 81-87, 2001. [14] E. Mannie, “Generalized Multi-Protocol Label Switching (GMPLS) Architecture,” IETF, RFC 3945, October 2004. [15] K. Kompella and Y. Rekhter, “OSPF Extensions in Support of Generalized Multi-Protocol Label Switching,” IETF, draft-ietf-ccamp-ospf-gmpls-extensions-12.txt, October 2003. [16] E. Oki et al., “Generalized Traffic Engineering Protocol,” IETF, draft-oki-ccamp-gtep-01.txt, October 2004. [17] D. Papadimitriou et al., “Shared Risk Link Groups Processing,” OIF, OIF2002.412, July 2002. [18] K. Aggarwal, J. Gupta, and K.Misra, “A Simple Method for Reliability Evaluation of a Communication System ,” Communications, IEEE Trans., 23, 563-566, May 1975.

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