1page.title=Network Interface Statistics Overview
2@jd:body
3
4<!--
5    Copyright 2013 The Android Open Source Project
6
7    Licensed under the Apache License, Version 2.0 (the "License");
8    you may not use this file except in compliance with the License.
9    You may obtain a copy of the License at
10
11        http://www.apache.org/licenses/LICENSE-2.0
12
13    Unless required by applicable law or agreed to in writing, software
14    distributed under the License is distributed on an "AS IS" BASIS,
15    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16    See the License for the specific language governing permissions and
17    limitations under the License.
18-->
19<p>In Android 4.0, statistics reported by Linux network interfaces are
20recorded over time, and are used to enforce network quota limits,
21render user-visible charts, and more.</p>
22<p>Each network device driver (Wi-Fi included) must follow the standard
23kernel device lifecycle, and return correct statistics through
24<code>dev_get_stats()</code>. In particular, statistics returned must remain
25strictly monotonic while the interface is active. Drivers may reset
26statistics only after successfully completing an <code>unregister_netdev()</code>
27or the equivalent that generates a <code>NETDEV_UNREGISTER</code> event for
28callbacks registered with <code>register_netdevice_notifier()</code> /
29<code>register_inetaddr_notifier()</code> / <code>register_inet6addr_notifier()</code>.</p>
30<p>Mobile operators typically measure data usage at the Internet layer
31(IP). To match this approach in Android 4.0, we rely on the fact that
32for the kernel devices we care about the <code>rx_bytes</code> and <code>tx_bytes</code>
33values returned by <code>dev_get_stats()</code> return exactly the Internet layer
34(<code>IP</code>) bytes transferred.  But we understand that for other devices it
35might not be the case. For now, the feature relies on this
36peculiarity. New drivers should have that property also, and the
37<code>dev_get_stats()</code> values must not include any encapsulation overhead
38of lower network layers (such as Ethernet headers), and should
39preferably not include other traffic (such as ARP) unless it is
40negligible.</p>
41<p>The Android framework only collects statistics from network interfaces
42associated with a <code>NetworkStateTracker</code> in <code>ConnectivityService</code>. This
43enables the framework to concretely identify each network interface,
44including its type (such as <code>TYPE_MOBILE</code> or <code>TYPE_WIFI</code>) and
45subscriber identity (such as IMSI).  All network interfaces used to
46route data should be represented by a <code>NetworkStateTracker</code> so that
47statistics can be accounted correctly.</p>
48