  Major wireless operators are nowadays facing network capacity issues in
striving to meet the growing demands of mobile users. At the same time,
3G-enabled devices increasingly benefit from ad hoc radio connectivity (e.g.,
Wi-Fi). In this context of hybrid connectivity, we propose Push-and-track, a
content dissemina- tion framework that harnesses ad hoc communication
opportunities to minimize the load on the wireless infrastructure while
guaranteeing tight delivery delays. It achieves this through a control loop
that collects user-sent acknowledgements to determine if new copies need to be
reinjected into the network through the 3G interface. Push-and-Track is
flexible and can be applied to a variety of scenarios, including periodic
message flooding and floating data. For the former, this paper examines
multiple strategies to determine how many copies of the content should be
injected, when, and to whom; for the latter, it examines the achievable offload
ratio depending on the freshness constraints. The short delay-tolerance of
common content, such as news or road traffic updates, make them suitable for
such a system. Use cases with a long delay-tolerance, such as software updates,
are an even better fit. Based on a realistic large-scale vehicular dataset from
the city of Bologna composed of more than 10,000 vehicles, we demonstrate that
Push-and-Track consistently meets its delivery objectives while reducing the
use of the 3G network by about 90%.
