  Zero-rating, the practice of not billing data traffic that belongs to certain
applications, has become popular within the mobile ecosystem around the globe.
There is an ongoing debate whether mobile operators should be allowed to
differentiate traffic or whether net neutrality regulations should prevent
this. Despite the importance of this issue, we know little about the technical
aspects of zero-rating offers since the implementation is kept secret by mobile
operators and therefore is opaque to end-users and regulatory agencies.
  This work aims to independently audit classification practices used for
zero-rating of four popular applications at seven different mobile operators in
the EU. We execute and evaluate more than 300 controlled experiments within
domestic and internationally roamed environments and identify potentially
problematic behavior at almost all investigated operators. With this study, we
hope to increase transparency around the current practices and inform future
decisions and policies.
