arXiv:1602.08563v3 Announce Type: cross Abstract: We initiate the study of a natural and practically relevant new variant of online caching where the to-be-cached items can have dependencies. We assume that the universe is a tree $T$ and items are tree nodes; we require that if a node $v$ is cached then the whole subtree $T(v)$ rooted at $v$ is cached as well. This theoretical problem finds an immediate application in the context of forwarding table optimization in IP routing and software-defined networks. We present an elegant online deterministic algorithm TC for this problem, and rigorously prove that its competitive ratio is $O(\mathrm{height}(T) \cdot k_{\mathrm{ONL}}/(k_{\mathrm{ONL}}-k_{\mathrm{OPT}}+1))$, where $k_{\mathrm{ONL}}$ and $k_{\mathrm{OPT}}$ denote the cache sizes of an online and the optimal offline algorithm, respectively. The result is optimal up to a factor of $O(\mathrm{height}(T))$. We complement the theoretical analysis with an empirical study on synthetic workloads and CAIDA backbone traffic, illustrating the trade-off between hit rate and cache-update cost.