In Android 4.0, statistics reported by Linux network interfaces are recorded over time, and are used to enforce network quota limits, render user-visible charts, and more.
Each network device driver (Wi-Fi included) must follow the standard
kernel device lifecycle, and return correct statistics through
dev_get_stats(). In particular, statistics returned must remain
strictly monotonic while the interface is active. Drivers may reset
statistics only after successfully completing an unregister_netdev()
or the equivalent that generates a NETDEV_UNREGISTER event for
callbacks registered with register_netdevice_notifier() /
register_inetaddr_notifier() / register_inet6addr_notifier().
Mobile operators typically measure data usage at the Internet layer
(IP). To match this approach in Android 4.0, we rely on the fact that
for the kernel devices we care about the rx_bytes and tx_bytes
values returned by dev_get_stats() return exactly the Internet layer
(IP) bytes transferred. But we understand that for other devices it
might not be the case. For now, the feature relies on this
peculiarity. New drivers should have that property also, and the
dev_get_stats() values must not include any encapsulation overhead
of lower network layers (such as Ethernet headers), and should
preferably not include other traffic (such as ARP) unless it is
negligible.
The Android framework only collects statistics from network interfaces
associated with a NetworkStateTracker in ConnectivityService. This
enables the framework to concretely identify each network interface,
including its type (such as TYPE_MOBILE or TYPE_WIFI) and
subscriber identity (such as IMSI). All network interfaces used to
route data should be represented by a NetworkStateTracker so that
statistics can be accounted correctly.