  In the backbone of large-scale networks, origin-to-destination (OD) traffic
flows experience abrupt unusual changes known as traffic volume anomalies,
which can result in congestion and limit the extent to which end-user quality
of service requirements are met. As a means of maintaining seamless end-user
experience in dynamic environments, as well as for ensuring network security,
this paper deals with a crucial network monitoring task termed dynamic
anomalography. Given link traffic measurements (noisy superpositions of
unobserved OD flows) periodically acquired by backbone routers, the goal is to
construct an estimated map of anomalies in real time, and thus summarize the
network `health state' along both the flow and time dimensions. Leveraging the
low intrinsic-dimensionality of OD flows and the sparse nature of anomalies, a
novel online estimator is proposed based on an exponentially-weighted
least-squares criterion regularized with the sparsity-promoting $\ell_1$-norm
of the anomalies, and the nuclear norm of the nominal traffic matrix. After
recasting the non-separable nuclear norm into a form amenable to online
optimization, a real-time algorithm for dynamic anomalography is developed and
its convergence established under simplifying technical assumptions. For
operational conditions where computational complexity reductions are at a
premium, a lightweight stochastic gradient algorithm based on Nesterov's
acceleration technique is developed as well. Comprehensive numerical tests with
both synthetic and real network data corroborate the effectiveness of the
proposed online algorithms and their tracking capabilities, and demonstrate
that they outperform state-of-the-art approaches developed to diagnose traffic
anomalies.
