Resilient optical networks are predominately designed to protect against single failures of fiber links.

Networks employ link protection to achieve fast recovery from link failures. While the first link failure can be protected using link protection, there are several alternatives for protecting against the second failure. This paper formally classifies the approaches to dual-link failure resiliency. One of the strategies to recover from dual-link failures is to employ link protection for the two failed links independently, which requires that two links may not use each other in their backup paths if they may fail simultaneously. Such a requirement is referred to as Backup Link Mutual Exclusion (BLME) constraint and the problem of identifying a backup path for every link that satisfies the above requirement is referred to as the BLME problem. This paper develops the necessary theory to establish the sufficient conditions for existence of a solution to the BLME problem. Solution methodologies for the BLME problem is developed using two approaches by: (1) formulating the backup path selection as an integer linear program; and (2) developing a polynomial time heuristic based on minimum cost path routing. The ILP formulation and heuristic are applied to six networks and their performance is compared to approaches that assume precise knowledge of dual-link failure. It is observed that a solution exists for all the six networks considered. The heuristic approach is shown to obtain feasible solutions that are resilient to most dual-link failures, although the backup path lengths may be significantly higher than optimal. In addition, the paper illustrates the significance of the knowledge of failure location by illustrating that network with higher connectivity may require lesser capacity than one with a lower connectivity to recover from dual-link failures.

The (Single-Edge) Replacement Paths problem is defined as follows: Given a weighted graph G(V, E), two nodes s and t, and the shortest path from s to t in G, compute the shortest path from s to t in the graph G\e i for 1 p. In other words, the single-edge replacement paths problem studies how a given s-t shortest path changes with the deletion of an edge lying on the path. We study a two-edge generalization of this problem for directed graphs, termed the Edge Pairs Replacement Paths problem: Given G, s, t, and (s, t) as defined above, compute the shortest path from s to t when two edges of pairs of edges of (s, t). We present an O(n ) algorithm for this problem, and establish an #(mn) lower bound in the path comparison model for shortest path algorithms which was introduced in [10].