Add segments service plug-in devref

This commit adds a document to devref that explains the implementation
of the segments plug-in

Change-Id: I80928eac7a6cb051c3d6532d83c9d9ecd328cd18

doc/source/contributor/internals/index.rst

@@ -61,3 +61,4 @@ Neutron Internals
    retries
    l3_agent_extensions
    live_migration
+   segments

doc/source/contributor/internals/segments.rst

@@ -0,0 +1,55 @@
+..
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+
+Segments extension
+==================
+
+Neutron has an extension that allows CRUD operations on the ``/segments``
+resource in the API, that corresponds to the ``NetworkSegment`` entity in the
+DB layer. The extension is implemented as a service plug-in.
+
+.. note:: The ``segments`` service plug-in is not configured by default. To
+   configure it, add ``segments`` to the ``service_plugins`` parameter in
+   ``neutron.conf``
+
+Core plug-ins can coordinate with the ``segments`` service plug-in by
+subscribing callbacks to events associated to the ``SEGMENT`` resource.
+Currently, the segments plug-in notifies subscribers of the following events:
+
+* ``PRECOMMIT_CREATE``
+* ``AFTER_CREATE``
+* ``BEFORE_DELETE``
+* ``PRECOMMIT_DELETE``
+* ``AFTER_DELETE``
+
+As of this writing, ``ML2`` and ``OVN`` register callbacks to receive events
+from the ``segments`` service plug-in. The ``ML2`` plug-in defines the
+callback ``_handle_segment_change`` to process all the relevant segments
+events.
+
+Segments extension relevant modules
+-----------------------------------
+
+* ``neutron/extensions/segment.py`` defines the extension
+* ``neutron/db/models/segment.py`` defines the DB models for segments and for
+  the segment host mapping, that is used in the implementation of routed
+  networks.
+* ``neutron/db/segments_db.py`` has functions to add, retrieve and delete
+  segments from the DB.
+* ``neutron/services/segments/db.py`` defines a mixin class with the methods
+  that perform API CRUD operations for the ``segments`` plug-in. It also has a
+  set of functions to create and maintain the mapping of segments to hosts,
+  which is necessary in the implementation of routed networks.
+* ``neutron/services/segments/plugin.py`` defines the ``segments`` service
+  plug-in.