We analyze the evolution of the global Internet interdomain routing system on AS, prefix and IP address level granularities, using snapshots of RouteViews BGP tables from 1997 to 2001. We introduce the notion of semiglobally routed prefixes, i.e., those present in the majority of backbone tables, and classify them into:

Using these distinctions we find that from 1999 to 2001 many measures of routing system complexity demonstrated stability in the form of slow growth, dynamic equilibrium, and occasional contraction.

We find that many net change measures reflect contributions of the opposite sign, and that true measures of variation, or churn, should take into account absolute magnitudes rather than their difference. Appearance and disappearance of prefixes, ASes and RouteViews peers, as well as status changes (an AS changing from transit to non-transit, or a prefix shifting from a standalone prefix to a root prefix) are instances of routing system churn. We find that the rates of long-term appearance and disappearance for IP addresses, prefixes, and ASes have comparable magnitudes and what is viewed as growth is sometimes just the "tip of the iceberg," with two rates almost cancelling each other out.

One advantage of using our notion of semiglobal prefixes is that they exhibit less churn than global prefixes (those prefixes common to all backbone tables) and as such allow for derivation of more robust macroscopic statistics about the routing system.

We study route prefix instability at a medium time granularity for late 2001 using 2-hour snapshots of BGP tables, and find that half of all prefix reannouncements (flips) are contributed by 1% of all ASes, with government networks, telecoms in developing countries, and major backbone ISPs at the top of the list of instability contributors. Small ASes (those that originate only a few prefixes into the global routing system) do not contribute more than their fair share of either route entries or churn. We conclude that during 1999-2001 many Internet metrics were stable, even though the sets that they measure were changing rapidly, and that the routing system's growth and instability are mostly caused by large and medium-sized ISPs.

About the Presenter
Andre Broido has been involved in algorithm development, data, and signal processing since the 1970s. In the mid-90s, his research focus shifted towards the Internet. In 1999-2002 he analyzed and monitored global Internet routing, topology, and round-trip times as CAIDA's Senior Statistician. Andre's papers and presentations can be found on CAIDA's website.