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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+Registering RouterInfo by L3 extension API
+==========================================
+
+Launchpad blueprint:
+https://blueprints.launchpad.net/neutron/+spec/router-factory-with-l3-extension
+
+Currently, most plugin implementations related to L3 override the *L3NATAgent*
+class itself for their own logic since there is no proper interface to
+extend the *RouterInfo* class. This adds unnecessary complexity for developers
+who just want to extend the agent mechanism instead of the whole RPC related
+to L3 functionalities.
+
+This spec introduces the *RouterFactory* class which acts on the factory for
+creating the *RouterInfo* class, and adds a new parameter to the L3 agent
+extension API which enables it to dynamically register *RouterInfo* to the
+factory. Now plugin developers can use the new extension API for their own
+specific router.
+
+
+Problem Description
+===================
+
+Current L3 agent implementation in Neutron consists of two parts in general.
+One is to implement an RPC Plugin API from Neutron server, and the other is to
+create ports, namespaces, and iptables rules using the data obtained from the
+RPC API on the server. To be more specific, the former is the *L3NATAgent*
+class and the latter is the *RouterInfo* class.
+
+The problem is that the two parts mentioned are now tightly coupled which means
+there is no clear way to extend each part individually. A lot of projects
+related to Neutron called networking-* [1]_, [2]_, [3]_ are making new L3
+agent classes on their own by extending the *L3NATAgent* class even though
+they did not modify the RPC mechanism but only changed the *RouterInfo*
+mechanism running on their server.
+
+Also, the current *RouterInfo* class does not have an abstract interface which
+makes it harder to extend the class for plugin developers. They have to find
+what functions and variables in *RouterInfo* are externally used in the L3
+agent to extend the *RouterInfo* behaviors.
+
+
+Proposed Change
+===============
+
+Today, *RouterInfo* is extended in several ways according to specific router
+features such as distributed, ha, and distributed + ha. This document proposes
+changing the L3 agent to have a new class called ``RouterFactory`` which has
+several pre-registered classes to extend the *RouterInfo* class with certain
+features. When it comes to creating an actual *RouterInfo* instance, the L3
+agent create a new instance from the ``RouterFactory`` following the features
+of the router. There is no functional change for existing code.
+
+*L3AgentExtensionAPI* now has a new parameter ``router_factory`` and a new
+function ``register_router``. A new abstract class called ``BaseRouterInfo``
+will be added. It will declare interfaces that are currently used externally.
+
+An L3 extension can register their own *RouterInfo* class which implements
+``BaseRouterInfo`` using the ``register_router`` API which has two parameters.
+
+(1) ``router``: RouterInfo declared in extension which overrides the one
+    pre-registered at ``RouterFactory``
+
+(2) ``features``: features of RouterInfo. Currently it should be one of the
+    below.
+
+    Features should be a list of strings describing router characteristics, and
+    the ordering does not matter since it is interpreted as a *set* internally.
+    (``['ha', 'distributed']`` and ``['distributed', 'ha']`` are the same)
+
+    - ``[]``: No feature. (e.g. ``LegacyRouter``)
+    - ``['distributed']``: Distributed router. (e.g. ``DvrEdgeRouter``,
+                           ``DvrLocalRouter``)
+    - ``['ha']``: HA router. (e.g. ``HaRouter``)
+    - ``['ha', 'distributed']``: Distributed HA router. (e.g.
+                                 ``DvrEdgeHaRouter``, ``DvrLocalRouter``)
+
+    Note that a router with the feature of ``['ha', 'distributed']`` can be
+    ``DvrLocalRouter`` when L3 agent mode is not ``dvr_snat`` [4]_.
+
+L3 extensions can override the *RouterInfo* class implemented in the Neutron
+codebase when it is initialized using the *initialize* function.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+* Yang Youseok <ileixe@gmail.com>
+
+Work Items
+----------
+
+* Add *RouterInfo* registration in *L3AgentExensionAPI* and pre-register
+  existing *RouterInfo* to *L3NATAgent*. [5]_
+* Add new exception named *RouterNotFoundInRouterFactory* to *neutron_lib*.
+  [6]_
+
+
+References
+==========
+
+.. [1] `networking-odl`:
+        https://github.com/openstack/networking-odl/blob/master/networking_odl/l3/l3_odl_v2.py#L47
+
+.. [2] `networking-ovn`:
+        https://github.com/openstack/networking-ovn/blob/master/networking_ovn/l3/l3_ovn.py#L50
+
+.. [3] `networking-calico`:
+        https://github.com/openstack/networking-calico/blob/master/networking_calico/plugins/calico/plugin.py#L26
+
+.. [4] https://docs.openstack.org/neutron/latest/configuration/l3-agent.html#DEFAULT.agent_mode
+
+.. [5] https://review.openstack.org/#/c/620349/
+
+.. [6] https://review.openstack.org/#/c/620348/