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Pre-Configuring IP-over-Optical Networks to
Handle Router Failures and Unpredictable Traffic
Murali Kodialam
T. V. Lakshman
Sudipta Sengupta
Bell Laboratories, Lucent Technologies, Holmdel, NJ, USA
Massachusetts Institute of Technology, Cambridge, MA, USA

Abstract— We consider the realization of traffic-oblivious routing in IP-over-Optical networks where routers are interconnected over a switched optical backbone, also called IP-over-OTN
(Optical Transport Network). The traffic-oblivious routing we consider is a scheme where incoming traffic is first distributed in a preset manner to a set of intermediate nodes. The traffic is then routed from the intermediate nodes to the final destination. This splitting of the routing into two-phases simplifies network configuration significantly [8], [17]. In implementing this scheme, the first and second phase paths are realized at the optical layer with router packet grooming at a single intermediate node only. Studies like [10] indicate that IP routers are 200 times more unreliable than traditional carrier-grade switches and average 1219 minutes of down time per year. Given this unreliability of routers, we consider how two-phase routing in IP-over-OTN can be made resilient against router node failures. We propose two different schemes for provisioning the optical layer to handle router node failures – one that is failure node independent and static, and the other that is failure node dependent and dynamic. We develop linear programming formulations for both schemes

and a fast combinatorial algorithm for the second scheme so as to maximize network throughput. In each case, we determine
(i) the optimal distribution of traffic to various intermediate routers for both normal (no-failure) and failure conditions, and (ii) provisioning of optical layer circuits to provide the needed inter-router links. We evaluate the performance of the two router failure protection schemes (in terms of throughput) and compare it with that of unprotected routing. For our experiments, we use actual ISP network topologies collected for the Rocketfuel project.

I. I NTRODUCTION
With the increasing use of the Internet for carrying realtime traffic such as VoIP traffic, it has become necessary for service providers to make their data networks highly reliable. Also, service providers have to engineer their networks to

handle multiple traffic patterns while avoiding congestion. This requires constant traffic monitoring, traffic forecasts and adapting the network routing to changing traffic conditions and to failures. This considerably increases the operational complexity.

Ideally, service providers would like to provision their
networks so that network operation is robust to changes in
traffic patterns (avoiding the need for frequent reconfiguration) while also accommodating failures in a fast and efficient
manner. The need to accommodate multiple traffic patterns
has led to interest in the hose traffic model [5] where the only traffic assumptions needed are the total amount of traffic entering and leaving each network ingress or egress port.
The actual traffic matrix itself need not be known. Several

routing and capacity allocation schemes for the hose model
have been proposed recently. An important scheme that allows the network to be statically configured so as to accommodate multiple traffic patterns is two-phase routing [8], [17]. Here traffic entering the network, instead of being directly sent to an egress-router, is first sent to an intermediate node, and from there is sent to the final egress-router. The first-phase distribution of traffic to the intermediate nodes is done in predetermined proportions that are depend on the intermediate nodes. Throughout this paper, we will refer to this scheme

as two-phase routing. The main contribution of this paper is in incorporating mechanisms for guaranteed QoS routing despite router failures while preserving the traffic-independence properties of two-phase routing.

In...
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