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| <title>Network Interface Statistics Overview</title> |
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| <p>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.</p> |
| <p>Each network device driver (Wi-Fi included) must follow the standard |
| kernel device lifecycle, and return correct statistics through |
| <code>dev_get_stats()</code>. In particular, statistics returned must remain |
| strictly monotonic while the interface is active. Drivers may reset |
| statistics only after successfully completing an <code>unregister_netdev()</code> |
| or the equivalent that generates a <code>NETDEV_UNREGISTER</code> event for |
| callbacks registered with <code>register_netdevice_notifier()</code> / |
| <code>register_inetaddr_notifier()</code> / <code>register_inet6addr_notifier()</code>.</p> |
| <p>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 <code>rx_bytes</code> and <code>tx_bytes</code> |
| values returned by <code>dev_get_stats()</code> return exactly the Internet layer |
| (<code>IP</code>) 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 |
| <code>dev_get_stats()</code> 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.</p> |
| <p>The Android framework only collects statistics from network interfaces |
| associated with a <code>NetworkStateTracker</code> in <code>ConnectivityService</code>. This |
| enables the framework to concretely identify each network interface, |
| including its type (such as <code>TYPE_MOBILE</code> or <code>TYPE_WIFI</code>) and |
| subscriber identity (such as IMSI). All network interfaces used to |
| route data should be represented by a <code>NetworkStateTracker</code> so that |
| statistics can be accounted correctly.</p> |
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