Ruepp, Sarah Renée

Stidsen, Thomas K.

Online Research Database In Technology

Shortcut Span ProtectionThomas Stidsen, IMM, Technical University of DenmarkSarah Ruepp, COM, Technical University of DenmarkKeywords: Protection, restoration overbuild, restoration complexity, restoration time, linear programming1. IntroductionDesigning resilient networks has been in focus of research and industry for some years now, resulting ina large variety of both well-known and experimental resilience provisioning mechanisms. Many of thesestudies focus on providing networks with the least amount of capacity possible to fulfil the demanded levelof protection.2. Mesh Protection MethodsTwo well-known methods for providing resilience in mesh networks are span protection and path protection.In span protection, the entire traffic is re-routed locally between the nodes adjacent to the failed span, meaningthat the end nodes of a connection are unaware of the failure. This results in a short notification time, butgenerally also means a longer protection path.In path protection, the affected connections are re-routed on an end-to-end basis, where an individual backuppath can be found for each connection. This approach results in high resource efficiency and does not nec-essarily result in a backup path of higher cost compared to the original working path. Restoring connectionsbetween their end-nodes requires considerably longer notification times than in the local span protection ap-proach. Since each affected connection can be re-routed individually, a large amount of complexity is addedto the network.A more recent method is termed local-to-egress or local backup dynamic protection, described in [1] and [2]respectively, where the traffic is re-routed between the upstream node adjacent to the failure and the egressnode of the connection. This combines the advantages of the two aforementioned methods, specifically theshort failure notification time and the freedom of finding a backup path that does not need resume the originalpath locally. The different protection methods are illustrated in Figure 1.Failure freeoperationLocal-to-egressLocal spanprotectionGlobal pathprotectionFigure 1: Protection methods for mesh networks3. Related WorkThe local-to-egress method was further studied in [3], where an arc-flow linear program to obtain optimalworking and backup paths in mesh networks was provided. In that model, each affected connection wasre-routed individually between the local failure adjacent node and the egress node. The model was applied tosmall networks and the finding was that the gain in capacity was rather small (order of a few percent).We question whether the small gain in capacity justifies adding the complexity of fully individual connectionre-routing in the local-to-egress protection method. Especially in core networks, where the trend is goingtowards having the complexity at the edge nodes of a network, the core routers should only have to deal withsimple decisions. Furthermore, with the prices for fiber (capacity) dropping [4], capacity usage will becomea less important factor for deciding which protection method should be employed, whereas complexity andspeed combined with the manageability of the method is expected to be given higher priority in the decisionprocess.4. ModelTo reduce the complexity of the protection method and the restoration time, we want to investigate a variationof the local-to-egress protection method, which we have termed, Shortcut Span Protection (SSP). In SSP, thetraffic is bundled between the failure adjacent node and the egress node if several affected connections havethe same destination node. The method is illustrated in Figure 2. The advantage of the proposed method liesin a less complex (and therefore faster) decision-making and routing process, resulting in less complex corenodes. The SSP is expected to require more capacity than path protection, but less than span protection.Figure 2: Shortcut Span Protection (SSP)We will now describe a Linear Programming (LP) model for the joint routing and protection problem for theSSP protection scheme. Given a network with N nodes indexed by i; j; k; l; q; r, a number of oriented linksL, indexed by (ij) and by (qr), indexing the normal links and the failed links respectively. The cost of thelink is given by the constant c(ij). The communication demand, i.e. the number of oriented circuits whichshould be established between the nodes k and l is given by the constant D(kl). The model is a so calledlink-path model and it contains 3 types of variables: x(kl)p 2 R+, the non-failure flow paths between nodek and node l, y(qr);lp 2 R+, the failure recovery flow from the start node q of the failed link qr to the endnode of the flow l, the required non-oriented capacity zfijg 2 R+. The non-failure paths are defined by theincidence matrix A(kl)p;(qr) 2 f0; 1g and the restoration paths B(qr);lp;(ij) 2 f0; 1g.Shortcut span protection link-path LP model minimize:∑fijgcfijg  zfijg (1)subject to:∑px(kl)p  D(kl) 8(kl); (2)2∑py(qr);lp −∑k∑pA(kl)p;(qr)  x(kl)p  0 8(qr); l; (3)zfijg −∑(kl)∑p[A(kl)p;(ij) + A(kl)p;(ji)]  x(kl)p (4)−∑l∑p[B(qr);lp;(ij) + B(qr);lp;(ji) ]  y(qr);lp −∑l∑p[B(rq);lp;(ij) + B(rq);lp;(ji) ]  y(rq);lp  0 8fijg; fqrg;x(kl)p ; y(qr);lp ; zfijg 2 R+ (5)The shortcut span protection LP model consists of an objective function (1), demand constraints (2), pro-tection constraints (3) and capacity constraints (4) and domain definitions (5). The objective function (1)measure the cost of the necessary capacity in the network. The demand constraint (2) ensures that the circuitsare established in the non-failure situation. The protection constraint (3) ensures that protection paths are as-signed to reroute communication in case of any single link failure. Finally constraint (4) ensure that enoughcapacity is assigned to the links to cover all single link failure situations.The shortcut span protection LP model corresponds closely to the model presented in [3], except it is for-mulated using paths. We will generate these paths using column generation. Afterwards, given both thenon-failure paths and the protection paths, we will solve a slightly different problem, where we only willallow one backup path for switch q, for oriented link (qr) to demand end node l. We will hence add a binaryvariable u(qr);lp , and the constraints given (6) (7) (8) below.y(qr);lp  M  u(qr);lp 8(qr); l; p (6)∑pu(qr);lp = 1 8(qr); l (7)u(qr);lp 2 f0; 1g (8)References[1] Authenrieth, A., Kirstaedter, A. ”Engineering End-to-End IP Resilience Using Resilience-DifferentiatedQoS,” IEEE Communications Magazine, pp. 50-57, January 2002.[2] Calle, E., Marzo, J., Urra, A. ”Protection performance components in MPLS networks,” ComputerCommunications, Vol. 27, pp. 1220-1228, Elsevier 2004.[3] Gruber, C.,G. ”A Comparison of Bandwidth Requirements of Path Protection Mechanisms,” ICN 2005,LNCS 3420, pp. 133-143, Springer-Verlag Berlin Heidelberg 2005.[4] SK Opto-Electronics Newsletter, ”Optical fiber supply meets expanding application, ” 15. February2003, Vol. 12.3

Shortcut Span Protection

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