y be more complex than the course modeling abstraction we adopted in [2]. Real AS relationships may depend on a peering point, prefix, and even time [2]. For example, ISPs that dominate one geographical region can provide transit for this region and receive transit for other regions, thus forming partial transit relationships with other ISPs. Such geographical interdependencies clearly lead to relationship cycles: three ASs active primarily in the US, Europe, and Asia, but also present in the two other regions, may form a cycle by providing transit to each other for their primary renterestingly, this kind of information-losing abstractions of different-type objects or relationships as nodes or links of the same type explain cycles in other flow networks. For example, in the economic network of the world trade web [10], cycles are present due to coarse categorizations of products and contractual relationships. Food webs (nodes are species and directed links show who eats whom) also have...
Xenofontas A. Dimitropoulos, M. Ángeles Ser