Data Usage Docs in Android 4.0 for Systems Integrators.
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+<!--
+ Copyright 2012 The Android Open Source Project
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+ Licensed under the Apache License, Version 2.0 (the "License");
+ you may not use this file except in compliance with the License.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+-->
+
+
+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.