Distributed algorithms for content placement in hierarchical cache networks

The growing popularity of mobile multimedia content and the increase of wireless access bitrates are straining backhaul capacity in mobile networks. A cost-effective solution to reduce the strain, enabled by emerging all-IP 4G and 5G mobile backhaul architectures, could be in-network caching of popular content during times of peak demand. Motivated by the potential benefits of caching in mobile backhaul networks, in this paper we formulate the problem of content placement in a hierarchical cache network as a binary integer programming problem. We provide a polynomial time solution when the link costs are induced by a potential and we propose a 2-approximation algorithm for the general case. The 2-approximation requires full information about the network topology and the link costs, as well as about the content demands at the different caches, we thus propose two distributed algorithms that are based on limited information on the content demands. We show that the distributed algorithms terminate in a finite number of steps, and we provide analytical results on their approximation ratios. We use simulations to evaluate the proposed algorithms in terms of the achieved approximation ratio and computational complexity on hierarchical cache network topologies as a model of mobile backhaul networks.