Hop-by-hop inter-domain routing protocols, such as BGP and IDRP, use independent route selection to realize domains ’ local policies. A domain chooses its routes based on path attributes present in a route. It is widely believed that these inter-domain routing protocols always converge. We show that there exist domain policies that cause BGP/IDRP to exhibit persistent oscillations. In these oscillations, each domain repeatedly chooses a sequence of routes to a destination. Complex oscillation patterns can occur even in very simple topologies. We analyze the conditions for persistent route oscillations in a simple class of interdomain topologies and policies. Using this analysis, we evaluate ways to prevent or avoid persistent oscillations in general topologies. We conclude that if a hop-by-hop inter-domain routing protocol allows unconstrained route selection at a domain, the protocol may be susceptible to route oscillations. Constraining route selection to a provably “safe ” procedure (such as shortest path) can reduce the number of realizable policies. Alternatively, a routing policy registry can help detect unsafe policies.

ion Approach By Polly Huang A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Computer Science) December 1999 Copyright 1999 Polly Huang 2 Table of Contents LIST OF FIGURES....................................................................................................................... 4 LIST OF TABLES......................................................................................................................... 6 DEDICATION ............................................................................................................................... 7 ACKNOWLEDGEMENTS .......................................................................................................... 8 ABSTRACT ................................................................................................................................... 9 INTRODUCTION ..................

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