Estimating routing symmetry on single links by passive flow measurements

The assumption of routing symmetry is often embedded into traffic analysis and classification tools. This paper uses passively captured network data to estimate the amount of traffic actually routed symmetrically on a specific link. We propose a Flow-Based Symmetry Estimator (FSE) -- a set of metrics to assess symmetry in terms of flows, packets and bytes, which disregards inherently asymmetrical traffic such as UDP, ICMP and TCP background radiation. This normalized metric allows fair comparison of symmetry across different links. We evaluate our method on a large heterogeneous dataset, and confirm anecdotal reports that routing symmetry typically does not hold for non-edge Internet links, and decreases as one moves toward core backbone links, due to routing policy complexity. Our proposed metric for traffic asymmetry induced by routing policies will help the community improve traffic characterization techniques and formats, but also support quantitative formalization of routing policy effects on links in the wild.