  The closed design of mobile devices -- with the increased security and
consistent user interfaces -- is in large part responsible for their becoming
the dominant platform for accessing the Internet. These benefits, however, are
not without a cost. Their operation of mobile devices and their apps is not
easy to understand by either users or operators. We argue for recovering
transparency and control on mobile devices through an extensible platform that
can intercept and modify traffic before leaving the device or, on arrival,
before it reaches the operating system. Conceptually, this is the same view of
the traffic that a traditional middlebox would have at the far end of the first
link in the network path. We call this platform ``middlebox zero'' or MBZ. By
being on-board, MBZ also leverages local context as it processes the traffic
and complements the network-wide view of standard middleboxes. We discuss the
challenges of the MBZ approach, sketch a working design, and illustrate its
potential with some concrete examples.
