tom

Added javadocs to the overview.html file to explain the overall software structure a bit better.

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<img src="doc-files/onos-tiers.png" alt="ONOS architecture tiers"><br>
ONOS architecture is strictly segmented into protocol-agnostic system core and
protocol-aware providers tiers.
<p>
ONOS architecture is strictly segmented into a <em>protocol-agnostic system
core</em> tier and the <em>protocol-aware providers</em> tier as shown in
the figure below:<br>
<img src="doc-files/onos-tiers.png" alt="ONOS architecture tiers">
</p>
<em>More information to come later...</em>
<p>
The <em>ONOS core</em> is responsible for tracking information about the
network environment and distributing it to the applications either
synchronously via query or asynchronously via listener callbacks. The
core is also responsible for persisting select state and synchronizing state
among the cluster peers.
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<p>
The <em>protocol-aware providers</em> are responsible for interacting with
the network environment using various control and configuration protocols
and supplying such sensory data to the core. Some providers may also need to
accept control edicts from the core and apply them to the environment
using the appropriate protocol-specific means.
</p>
<p>
The following diagram describes a general structure of each ONOS subsystem:
<br>
<img src="doc-files/onos-subsystem.png" alt="ONOS subsystem structure"><br>
For example, the device-subsystem comprises of a core
{@link org.onlab.onos.net.trivial.device.impl.SimpleDeviceManager DeviceManager},
which exposes a north-bound
{@link org.onlab.onos.net.device.DeviceService} through which applications or other core components
can learn about the global infrastructure device inventory and through
which they can also subscribe for asynchronous {@link org.onlab.onos.net.device.DeviceEvent}
notifications via the {@link org.onlab.onos.net.device.DeviceListener} mechanism. A set of
administrative actions can be performed via {@link org.onlab.onos.net.device.DeviceAdminService},
e.g. setting mastership role, removing a decommissioned device.
</p>
<p>
On the south-bound side, the core {@link org.onlab.onos.net.trivial.device.impl.SimpleDeviceManager DeviceManager}
exposes a {@link org.onlab.onos.net.device.DeviceProviderRegistry} through which any number of
{@link org.onlab.onos.net.device.DeviceProvider} entities can register and in turn obtain a
{@link org.onlab.onos.net.device.DeviceProviderService}. Device and port information can then be
supplied to the core by each provider through the provider service issued
to them. When a provider unregisters, its {@link org.onlab.onos.net.device.DeviceProviderService}
will be invalidated and can no longer be used for interacting with the
core.
</p>
<p>
Within the core, the tasks of indexing, persisting and synchronizing the
global device and port state with the cluster peers falls on the
{@link org.onlab.onos.net.trivial.device.impl.SimpleDeviceStore DeviceStore}.
</p>
<p>
Similar structure applies to the link subsystem, host subsystem and others.
</p>
<p>
<em>More information to come later...</em>
</p>
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