Quantifying AS-level routing policy changes

To study Internet´s routing behavior on the granularity of Autonomous Systems (ASes), one needs to understand inter-domain routing policy. Routing policy changes over time, and may cause route oscillation, network congestion, and other problems. However, there are few works on routing policy changes and their impact on BGP´s routing behaviors. In this paper, we model inter-domain routing policy as the preference to neighboring ASes, noted as neighbor preference, and propose an algorithm for quantifying routing policy changes based on neighbor preference. As a further analysis, we study the routing policy changes for the year of 2012, and find that generally an AS may experience a routing policy change for at least 20% prefixes within 6 months. An AS changes its routing policy mainly by exchanging two neighboring ASes´ preference. In most cases, an AS changes a stable fraction of its prefixes´ routing policy, but non-tier1 ASes may endure a large scale routing policy changing event. We also analyse the main reasons of routing policy changes, and exclude the possibilities of AS business relationship changes and topology changes.