Hic Sunt Proxies: Unveiling Proxy Phenomena in Mobile Networks

Over the years middleboxes have established themselves as a solution to a wide range of networking issues, progressively changing network landscape and turning the end-to-end principle into a reminder of an Arcadian age of the Internet. Among them, proxies have found breeding ground especially in mobile networks that, moreover, have become the most popular way to access the Internet.In this paper, we present Mobile Tracebox, an Android measurement tool, and describe how its methodology, coping with the lack of privileges of mobile devices, can not only detect proxies but also characterize different facets, from their transport layer behavior to their location inside the network. Data collected from a crowdsourced deployment over more than 90 carriers and 350 Wi-Fi networks contributes to describe the potential of the tool and to draw a panorama of proxies across mobile networks. Our study confirms their prevalence and reveals that their scope is not limited to HTTP but can include several TCP services and even non standard ports. We detail the different implementations observed and delve into specific aspects of their configuration, like the initial Receive Window, the Window Scale factor or the set of Options supported, to understand how proxies can affect performance or obstruct extensions. Finally, we focus on fingerprinting and attempt to draw a dividing line between packet modifications performed by proxies and those performed by other classes of middleboxes.