Positioning Relay Nodes in ISP Network

To increase reliability and robustness of mission-critical services in the event of network failures, it is often desirable and beneficial to take advantage of path diversity provided by the network topology. One way of achieving this inside a single Autonomous System (AS) is to use two paths between every Origin-Destination (OD) pair. One path is the default path defined by the intra-domain routing protocol running within the AS; the other path is defined as an overlay path that passes through a strategically placed relay node inside the AS. The key question then is how to place such relay nodes inside an AS, which is the focus of this work. We propose a simple greedy algorithm to find the number and positions of relay nodes such that every OD pair has an overlay path going through a relay node that is disjoint from the default path. When it is not possible to find completely disjoint overlay paths, we allow overlay paths to have overlapped links with default paths. Since overlapped links diminish the robustness of overlay paths against a single point of failure, we introduce the notion of penalty for partially disjoint paths. We apply our algorithm on an operational tier-1 ISP network and demonstrate that our method increases network-wide resiliency against a single link failure. Based on real failure scenarios obtained from the ISP network and hypothetical traffic matrix, we demonstrate that the relay nodes selected by our algorithm provide complete protection against 75.3% of failure events and allow less than 1% of traffic to be affected for 92.8% of failure events.