Registering RouterInfo by L3 extension API

Launchpad blueprint:

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,


    • ['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.



Work Items

  • Add RouterInfo registration in L3AgentExensionAPI and pre-register existing RouterInfo to L3NATAgent. [5]

  • Add new exception named RouterNotFoundInRouterFactory to neutron_lib. [6]