  The next generation of networks must support billions of connected devices in
the Internet-of-Things (IoT). To support IoT applications, sources sense and
send their measurement updates over the Internet to a monitor (control station)
for real-time monitoring and actuation. Ideally, these updates would be
delivered at a high rate, only constrained by the sensing rate supported by the
sources. However, given network constraints, such a rate may lead to delays in
delivery of updates at the monitor that make the freshest update at the monitor
unacceptably old for the application.
  We propose a novel transport layer protocol, namely the Age Control Protocol
(ACP), that enables timely delivery of such updates to monitors, in a
network-transparent manner. ACP allows the source to adapt its rate of updates
to dynamic network conditions such that the average age of the sensed
information at the monitor is minimized. We detail the protocol and the
proposed control algorithm. We demonstrate its efficacy using extensive
simulations and real-world experiments, which have a source send its updates
over the Internet to a monitor on another continent.
