6.4 The Internet Autonomous System Topology

The Internet's AS topology is a widely studied representation of the Internet at a particular scale. An AS represents a single network that can apply its own operational and peering policy. An Internet service provider (ISP) may use one or more ASes. The Internet contains over 30,000 ASes, each in a set of relationships with its neighbors, who are either its customers, providers, or peers. In the Internet core there is a full mesh formed between the ASes of the various tier-1 ISPs. However, at the edge there are a huge number of smaller ISPs and customer networks which connect through upstream providers and local public exchange points. These smaller ISPs and customer networks may have only one upstream provider, or may have many for resilience and performance reasons. In addition, the Internet constantly evolves new networks are added, old ones disappear, and existing ones grow and merge.

Links between ASes depend on business relationships which can and do change, sometimes rapidly, making any interpretation of the Internet as a static structure inaccurate. This rich and dynamic structure makes it difficult to provide either a single, representative topological model, or a single graph metric that captures all characteristics of any topology. However, such a metric would make it possible to generate realistic synthetic topologies improving the accuracy of Internet-wide protocol simulations, and perhaps enabling the prediction of the ...

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