Testing for Traffic Differentiation with ChkDiff: The Downstream Case

In the past decade it has been found that some Internet operators offer degraded service to selected user traffic by applying various differentiation techniques. If from a legal point of view many countries have discussed and approved laws in favor of Internet neutrality, confirmation with measuring tools for even an experienced user remains hard in practice. In this paper we extend and complete our tool ChkDiff, previously presented for the upstream case, by checking for shaping also on the user's downstream traffic. After attempting to localize shapers at the access ISP on upstream traffic, we replay downstream traffic from a measurement server and analyze per-flow one-way delays and losses, while taking into account the possibility of multiple paths between the two endpoints. As opposed to other proposals in the literature, our methodology does not depend on any specific Internet application a user might want to test and it is robust to evolving differentiation techniques that alter delays or induce losses. We provide here a detailed description of the downstream tool and a validation in the wild for wired, WiFi and 3G connections.