Survivability in IP over WDM networks YINGHUA YE and SUDHIR DIXIT Nokia Research Center, Burlington, Massachusetts

Outline Survivability strategies ․ Protection and restoration Survivable routing algorithms ․ Conventional survivable routing ․ Suurballe`s Algorithms ․ Joint Working and Protection Routing Approach

Survivability strategies Survivability can be classified as protection and restoration. Protection mechanism are designed against a single failure event, whereas restoration can be used against multifailure events.

Survivability strategies - Protection Protection is preprovisioned failure recovery, where a secondary path is computed at the same time as the primary path. ․ otherwise, the connection request is rejected. Two forms of protection mechanism : ․ Dedicated ․ Shared

Survivability strategies - Protection (contd.) Dedicated ․ use 100% of the protection capacity overbuilt for dedication to the primary connection.

Survivability strategies - Protection (contd.) The main advantage of dedicated backup reservation method is shorter restoration time. ․ However, this method has a defect of low resource utilization. For better resource utilization, multiplexing techniques have been studied extensively.

Survivability strategies - Protection (contd.) Shared protection scheme

Survivability strategies - Protection (contd.) Issue of shared protection scheme : ․ The reversion from secondary path to corresponding primary path after failure has been restored. The shared protection scheme requires the primary paths whose secondary paths share the resources cannot incur the same risk. ․ SRLG (Shared Risk Link Group)

Survivability strategies - Protection (contd.) Example of SRLG

Survivability strategies - Protection (contd.) Protection architectures : ․ 1+1 ․ 1 : 1 - two types of 1 : 1 protection - The 1 : 1 protection can be extended to M : N protection. 1+1 architecture is faster and simpler than 1 : 1 but at the expense of lower network utilization.

Survivability strategies - Restoration Restoration is after-the-fact dynamic signaled recovery. ․ centralized ․ distributed In both restoration approaches, if an alternative path cannot be discovered for a broken connection, that connection is blocked. ․ No recovery guarantee and long restoration time.

Survivable routing algorithms Depending on the different traffic patterns, many researchers have proposed different approaches to solve the RWA problem. ․ SLE (Static Lightpath establishment) - RWA objective : minimize the wavelength utilization. ․ DLE (Dynamic Lightpath establishment) - RWA objective : minimize the request blocking probability. - two choice of DLE : 1. hop-by-hop 2. explicit routing

Survivable routing algorithms - Cost function There are many definitions of link cost in the literature, which can be classified into topology based, resource use based, or topology and resource use integrated. An integrated link cost function is as follows : resource weightshop count cost

Survivable routing algorithms - Cost function Generally, for the working path or dedicated protection path, the path cost is the sum of the link cost at each hop. However, for the shared protection path, there are two possible scenarios. ․ the protection path is defined similar to the case of working path ․ if shared protection is considered, the protection path cost is more related to resource consumption than to link cost.

Survivable routing algorithms - Cost function The working path cost CW and the protection path cost CP are defined as follow : φ(l(i,j), α) equals to l(i,j) when p has noλsharing with other protection path(s). Otherwise, 0<= φ(l(i,j), α) < l(i,j)

Survivable routing algorithms - Conventional survivable routing The method adopts a two-step algorithm : 1. selects a route with minimum cost for the working path at first, and then removes the links along the working path. 2. selects a route with minimum cost for the protection path. Although both the working and protection paths are selected separately with minimum cost, the sum of the working and protection costs may not be minimal.

Survivable routing algorithms - Conventional survivable routing (contd.) Sd Total cost from S to D = 11 Total cost from S to D = 10

Survivable routing algorithms - Suurballe`s Algorithms To overcome the shortcomings of the conventional approach, Suurballe`s algorithms were developed to search for the shortest pairs of disjointed paths. Unfortunately, the tasks of this algorithm take time to finish, and as the number of links in the network increases, this time can be substantial.

Survivable routing algorithms - Suurballe`s Algorithms (contd.) Step 1: Find the shortest path SP (by Dijkstra's algo.) from source S to destination D. Label each node by d(i), which is defined as the shortest distance from source node to node i. Step 2: Reverse all arcs along the shortest path SP, and multiply their cost with – 1. Step 3: Modify the cost of all arcs according to the following formula: ce´ (i-> j) = ce (i->j) + d(i) – d(j), so as to get a non-negative arc cost. The modified graph is denoted by G´. Step 4: Find the shortest path SP´ in the modified graph G´. Step 5: Map the shortest path of SP´ to G and eliminate the common arcs of SP and SP´ in G to obtain the two desired paths with minimum total cost.

Survivable routing algorithms - Suurballe`s Algorithms (contd.) Step5:elimination total cost =4+3=7

Survivable routing algorithms - Joint Working and Protection Routing Approach To simplify computation and make the disjoint path algorithm more practical, a joint lightpath selection has been proposed.

Survivable routing algorithms - Joint Working and Protection Routing Approach (contd.) n2 n4n3 n1 d S 1 2

Survivable routing algorithms - Joint Working and Protection Routing Approach (contd.) Step1 : Compute K candidate routes with Wi representing the ith (1<=i<=K) route and CWi as the cost of Wi. W1 : S->n1->n2->D, CW1=4 W2 : S->n1->n4->D, CW2=3 W3 : S->n3->n4->D, CW3=3 W4 : S->n3->D, CW4= n2 n4n3 n1 d S 1 2

Survivable routing algorithms - Joint Working and Protection Routing Approach (contd.) Step2 : For Wi(1<=i<=K), compute an edge-disjoint route, represented as Pi and the cost of Pi denoted as CPi. P1 : S->n3->n4->D, CP1=3 S->n3->D, CP1=2 P2 : S->n3->D, CP2=2 P3 : S->n1->n2->D, CP3=4 P4 : S->n1->n2->D, CP4=4 S->n1->n4->D, CP4= n2 n4n3 n1 d S 1 2

Survivable routing algorithms - Joint Working and Protection Routing Approach (contd.) Step3: Find h such that CWh + CPh = min (CWi + CPi), 1<=i<=K. Step4: Select Wh as the working path and Ph as the protection path. CW1 + CP1 = 7 or 6 CW2 + CP2 = 5 CW3 + CP3 = 7 CW4 + CP4 = 6 or n2 n4n3 n1 d S 1 2