1page.title=Overview
2@jd:body
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19<p>The per-application/delegated data usage monitoring and tracking
20functionality relies on the xt_qtaguid module in the android-3.0 Linux
21kernel (<code>kernel/net/netfilter/xt_qtaguid</code>). The socket tagging
22functionality in the framework (<code>system/core/libcutils/qtaguid.c</code>)
23relies mainly on the existence of <code>/proc/net/xt_qtaguid/ctrl</code>
24interface exported by the <code>xt_qtaguid</code> kernel module.</p>
25<p>The <code>quota2</code> netfilter module (originally part of <code>xtables-addons</code>)
26allows the functionality to set named quota limits and was extended to
27support notifying userspace when certain limits are reached. Once the
28quota limit is reached, the <code>quota2</code> module discards all subsequent
29network traffic. The framework can also specify additional rules to
30restrict background data traffic for an application (refer to
31<code>com.android.server.NetworkManagementSocketTagger.setKernelCounterSet</code>
32and
33<code>android.net.NetworkPolicyManager.POLICY_REJECT_METERED_BACKGROUND</code>).</p>
34<h1 id="how-does-it-work">How does it work?</h1>
35<p>The <code>qtaguid</code> netfilter module tracks the network traffic on a
36per-socket basis for every application using the unique UID of the
37owning application.  There are two tag components associated with any
38socket in the system. The first is the UID which uniquely identifies
39the application which is responsible for the data transfer (Linux
40allows the ability to ascribe the ownership of each network socket to
41the UID of the calling application). The second tag component is used
42to support additional characterization of the traffic into application
43developer specified categories. Using these application level tags, an
44application can profile the traffic into several sub-categories.</p>
45<p>In the case of applications that provide network data transfer as a
46service, such as the download manager, media streaming service, etc,
47it is possible to attribute the ownership of the network data transfer
48to the UID of the requesting application using the
49<code>TrafficStats.setThreadStatsUid()</code> function call. The caller must hold
50the “<code>android.permission.MODIFY_NETWORK_ACCOUNTING</code>” permission to
51re-assign the ownership of the network traffic.</p>
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