Handle Router Failures and Unpredictable Trafﬁc
T. V. Lakshman
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 , . 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  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  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 , . 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.