ISP networks have become a critical infrastructure in our society. Traffic in these networks is growing and is increasingly dominated by a small number of large CDNs connecting at multiple locations. Simultaneously, the networks are becoming more flexible, in terms of routing, CDN user mapping, and also regarding the IP topology: emerging optical technologies allow to flexibly reconfigure the network. This paper studies the potential gains of these reconfiguration flexibilities. The idea is to make CDN-ISP infrastructure demandaware, that is, to re-optimize it towards the changing end-user demands over time. We present an optimization framework and conduct an extensive evaluation using data from a large European ISP. We find that such a reconfigurable infrastructure has indeed a high potential: by leveraging spatial and diurnal traffic patterns, the efficiency of ISP networks and CDNs is improved significantly. Specifically, the required backbone capacity is reduced by 15% while reducing path lengths by 30%, on average and during the critical peak hour. Moreover, such infrastructures can leverage re-optimizations during specific events, like the COVID-19 pandemic, and under link failures. We optimistically assume a cooperative environment of ISPs and CDNs, and we conclude by discussing trends that foster the identified benefits in practice.

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