1<!-- 2 Copyright 2012 The Android Open Source Project 3 4 Licensed under the Apache License, Version 2.0 (the "License"); 5 you may not use this file except in compliance with the License. 6 You may obtain a copy of the License at 7 8 http://www.apache.org/licenses/LICENSE-2.0 9 10 Unless required by applicable law or agreed to in writing, software 11 distributed under the License is distributed on an "AS IS" BASIS, 12 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 See the License for the specific language governing permissions and 14 limitations under the License. 15--> 16 17 18In Android 4.0, statistics reported by Linux network interfaces are 19recorded over time, and are used to enforce network quota limits, 20render user-visible charts, and more. 21 22Each network device driver (Wi-Fi included) must follow the standard 23kernel device lifecycle, and return correct statistics through 24`dev_get_stats()`. In particular, statistics returned must remain 25strictly monotonic while the interface is active. Drivers may reset 26statistics only after successfully completing an `unregister_netdev()` 27or the equivalent that generates a `NETDEV_UNREGISTER` event for 28callbacks registered with `register_netdevice_notifier()` / 29`register_inetaddr_notifier()` / `register_inet6addr_notifier()`. 30 31Mobile operators typically measure data usage at the Internet layer 32(IP). To match this approach in Android 4.0, we rely on the fact that 33for the kernel devices we care about the `rx_bytes` and `tx_bytes` 34values returned by `dev_get_stats()` return exactly the Internet layer 35(`IP`) bytes transferred. But we understand that for other devices it 36might not be the case. For now, the feature relies on this 37peculiarity. New drivers should have that property also, and the 38`dev_get_stats()` values must not include any encapsulation overhead 39of lower network layers (such as Ethernet headers), and should 40preferably not include other traffic (such as ARP) unless it is 41negligible. 42 43The Android framework only collects statistics from network interfaces 44associated with a `NetworkStateTracker` in `ConnectivityService`. This 45enables the framework to concretely identify each network interface, 46including its type (such as `TYPE_MOBILE` or `TYPE_WIFI`) and 47subscriber identity (such as IMSI). All network interfaces used to 48route data should be represented by a `NetworkStateTracker` so that 49statistics can be accounted correctly. 50