Tags represent one of the metrics the data usage counters will be
tracked against. By default, and implicitly, a tag is just based on
the UID. The UID is used as the base for policing, and cannot be
ignored. So a tag will always at least represent a UID (uid_tag). A
tag can be explicitly augmented with an "accounting tag" which is
associated with a UID. User space can use
TrafficStats.setThreadStatsTag() to set the acct_tag portion of the
tag which is then used with sockets: all data belonging to that
socket will be counted against the tag. The policing is then based on
the tag's uid_tag portion, and stats are collected for the acct_tag
portion separately.
Without explicit tagging, the qtaguid module will assume the
default_tag: {acct_tag=0, uid_tag=10003}
a: {acct_tag=1, uid_tag=10003}
b: {acct_tag=2, uid_tag=10003}
c: {acct_tag=3, uid_tag=10003}
a, b, c… represent explicit tags associated with specific sockets.
default_tag (acct_tag=0) is the default accounting tag that contains
the total traffic for that uid, including all untagged
traffic, and is typically used to enforce policing/quota rules.
These tags can be used to profile the network traffic of an application into separate logical categories (at a network socket level). Such tags can be removed, reapplied, or modified during runtime.
The qtaguid module has been implemented on kernel/common branch of android-3.0