Restructure L3 Agent

Author:Carl Baldwin <>

The L3 agent is implemented mostly in a single python file. At current count, this file is just over 2,000 lines of code [1]. Most of the functionality is provided by the L3NATAgent class which comprises about 75% of the file. This class handles everything from handling RPC messages down to sending gratuitous arp for newly added addresses on the interfaces inside the routers’ namespaces. This structure makes the agent very difficult to extend and modify. This is a bit of technical debt. Paying it down will help enable the development of new functionality.


Problem Description

As mentioned in the introductory paragraph, the L3 agent has gotten out of hand. The structure of this code has made it difficult to extend and develop new features. Following is a list of responsibilities taken care of in the file and mostly in the L3NATAgent class within that file.

  • Defines the L3PluginApi
  • Manages link local addresses for the DVR fip namespace
  • Defines the RouterInfo, basically a big struct of data about a router
  • Handles router update messages from RPC in a queue
  • Handles periodic synchronization of all routers
  • L3NATAgent
    • Manages namespace lifecycle
    • Handles router addition removal
    • Runs metadata proxy in each router
    • Very large method called process_router
      • snat_rules, dnat_rules
      • floating_ip_address
      • external gateway
      • internal network interfaces
      • ipv6 support
      • cleanup of stale interfaces
      • static routes
      • HA router keepalive
    • DVR routers
      • rtr_2_fip
      • dvr floating ips
      • snat namespace handling
      • arp entries
    • routes_updated
    • _process_routers_loop
    • _sync_routers
    • gratuitous arp
  • L3NATAgentWithStateReport

Code for DVR and HA routers is mingled throughout. There is a lot of “if router[‘distributed’]” this and “if ri.is_ha” that.

There is no clear strategy for resource life-cycle management of resources like namespaces and devices. Most of it has evolved over time as problems with the initial implementation have been found. Such problems are usually around them not being deleted when they should be gone.

Proposed Change


This will not be a rewrite of the L3 agent from scratch. Starting this project as a Herculean effort designed to land as a single patch is a sure recipe for failure. Much of this work will be pure refactoring but not all of it. Work will be posted for review early and often. Dependencies between patches will be avoided when possible. Each patch will stand on its own as a reasonably reviewable improvement to the existing code base.

Proper separation of concerns is a goal however, this will be done in steps. We’ll start with the high level separation of the router abstraction from the L3 agent.

L3 Agent

The L3 agent will be responsible for listening for updates from RPC, queuing the updates for processing by a worker. It will continue to oversee the set of routers which are managed by the agent. If namespaces are enabled, this could be a large set of routers. If namespaces are not enabled, this will be a single router. (_process_routers, _sync_routers, routes_updated, _router_added/removed)

The agent will still manage external networks available to routers on the agent.

The L3PluginApi class will remain as it is in

The agent will retain the L3NATAgentWithStateReport capability.


A new router class will be introduced. A lot of functionality currently handled by the L3 agent – especially the functionality in the process_routers method – will be encapsulated by this new class. The current RouterInfo class will move under this abstraction. This class will obsolete and replace the RouterInfo class.

The router class will be more than just a struct with data about the router. It will be a full-fledged class that is capable of handling the implementation of the router. It needs a clear and uncomplicated python API defined.

As of the Juno release, there are three kinds of routers available. These are distributed, highly available, and legacy routers. A new router class hierarchy will be added to encapsulate the details of each available type of router. The appropriate class will be loaded when the router instance is first created.

Kilo or beyond will see the addition of a fourth type of router which combines DVR and HA routers. Adding this fourth type is out of the scope of this blueprint. However, adding this new type of router should be relatively easy after this blueprint has been completed by creating a new class type which combines the functions of the separate base classes. This new classes should be written in a way which efficiently makes use of the existing code in the two base classes. Any additional complexity in this module should only exist to work out any coordination which needs to happen between the two classes.

The above uses inheritence to encapsulate the details of the various kinds of routers with an abstract base router serving as the base class and the others implemented as sub-classes. The HA DVR type of router would then use multiple inheritence. Following is a dot representation of what I imagine the hierarchy will be. Notice that LegacyRouter is not a base class. This reflects the fact that “DistributedRouter is a LegacyRouter” is not a true statement. Also, there are two DVR classes. This reflects the fact that non-network nodes have the distributed part of a DVR and network nodes have the central part which builds from the distributed part:

digraph inheritence {
  "LegacyRouter" -> "Router"
  "DistributedRouter" -> "Router"
  "DistributedRouterCentral" -> "DistributedRouter"
  "HARouter" -> "Router"
  "HADistributedRouter" -> "HARouter"
  "HADistributedRouter" -> "DistributedRouterCentral"

Given that HA and DVR are properties of individual routers and not properties of the deployment, we will need to pay attention to the migration path from one to another. The code should fully expect that a router can change from one type to another and have the capability to handle it by changing the class used for a router. I expect that the router should be functional with its new type and that any namespaces, devices, or other resources that are no longer necessary after the router changes type will be cleaned up. The clean up will be handled by the resource lifecycle pattern described in the Resource Lifecycle section.

The very long _process_router method needs to be refactored with this. The following responsibilities are handled here. Eventually, these will be abstracted behind other interfaces (like an iptables abstraction) but that work may not be completely done as part of this effort. At a high level, the refactoring of this method will separate concerns like plugging interfaces to networks from routing responsibilies.

  • snat_rules, dnat_rules
  • floating_ip_address
  • external_gateway_added
  • internal network added
  • static routes


There are a few services implemented in the L3 agent in various ways. This blueprint will add a simple service driver model to support decoupling these services from the L3 agent class and its inheritence hierarchy. As stated before, inheritence will not be used to integrate these services. Each of the services will be moved to a new service specific module

Essentially, the agent will be a basic container which loads services as classes. The routing service orchestrates the workflow for services by dispatching router events to each of the known services sequentially. For this blueprint, the dispatching will likely be implemented as a simple method call to a common service interface. This can be expanded to support a more pluggable model as a follow-on effort.

The services will have a reference to the router in order to access L3 function such as adding/removing NAT rules and opening ports.

I don’t intend to make any significant changes to the device driver models that are implemented in the FW and VPN services in the scope of this blueprint. I don’t expect this effort to have any effect on the configuration of services. Backward compatibility will be actively preserved. This may involve leaving stubs in place for the VPNAgent and others to load a VPN enabled L3 agent.

Existing integration tests will be modified to work with the new structure.

The intent here will not be to make a model that is everything to everyone. That is out of the scope of this blueprint. The intent is to iteratively develop an interface that will work for the following services which are already integrated with the L3 agent. The goal is to reduce coupling and pave the way for a more sophisticated model which may be needed in the future. They will be tackled in the order listed and the interface will evolve to support them all.

  1. Metadata Proxy
  • The easiest one. Low-hanging fruit.
  1. FWaaS
  • Want to remove it as a super-class of L3NATAgent
  1. VPNaaS
  • Want to remove it as a sub-class of L3NATAgent

The first step is to create a service abstract class, and then sub-classes for the various services to use these as observers to the L3 agent. The base class would have no-op methods for each action that the L3 agent could notify about, and the child classes would implement the ones they’re interested in. Each service will register as an observer.

Currently, the L3 agent (and VPN agent) load the device drivers for services. What can be done in this first step, is, instead of doing the load, a service object can be created. This object would do the loading and register with the L3 agent for notifications.

The child services’ notification handlers will be populated by moving the code in the various agent classes into the new service child classes, and adapt as needed.

Anything more complicated than this should be considered out of the scope of this blueprint.

Some guidelines for this work:

  1. We don’t need the service abstract class to be perfectly and completely defined in advance. I intend to do this iteratively tackling the services in the order listed above. This means that we don’t review the changes to decouple the metadata proxy with the needs of the VPN agent in mind.
  2. This initial decomposition should be done without changing any configuration or other deployment details. This might mean that we leave, for example, a tiny stub of a VPNAgent class in place.
  3. Initially, the services will get an L3 agent passed in on create, but in the as the blueprint progresses, a router instance can be passed to the service.

DVR Router Class

Everything related to the floating IP namespace that was added for DVR should be encapsulated in a driver for plugging a router in to an external network and handle floating ip setup. This includes the LinkLocalAllocator, dvr specific floating ip processing, fip namespace management, connection of router to fip (rtr_2_fip, fip_2_rtr), _create_dvr_gateway, and the management of proxy arp entries.

HA Router Class

This encapsulation will hide the details related to starting keepalived and creating and using interfaces needed for the HA network on which it communicates.

Resource Lifecycle

The major problem here is that resources are often left lying around beyond their useful lifecycle. Assumptions were made about the reliable availability of the agent, guaranteed ordering and delivery of RPC messages, and other unrealistic guarantees. The new design will account for problems in these areas. No assumptions will be made. This will result in a more robust implementation.

The problem that we’ve had with this is that the agent fails to cleanup resources when they should no longer exist. To address this, I’m thinking of something that supports the following pattern using namespaces as an example:

if full_sync:
    with namespace_manager.prepare_to_clean_up_stale() as nsm:
        for router in all_active_routers:

The __enter__ and __exit__ methods should work together to discover stale namespaces and then clean them up. I’m thinking maybe a namespace object should hold a weak reference to the router that occupies it. When the weak ref goes stale then the namespace can be removed. This pattern is not too different from what exists in the code now since some earlier refactoring that I did. However, this effort will formalize the pattern and abstract it from the rest of the code. Code has been started to illustrate this pattern [2].


The pattern can be applied to other resources such as interfaces inside of a namespace. We have had problems ensuring that those get removed when they are no longer useful as well. For devices and other resources in a router, the active resources would all be marked each time a router is processed. Stale resources are then identified and removed.

There has been a problem with namespaces which are persistently difficult to delete due to a problem in the version of iproute in use on the system [3] and [4]. There really is nothing that can be done to remove these except to reboot the machine. However, the new implementation of resource lifecycle management will hold a set of namespaces that it has tried to delete. If the deletion fails, it will skip this deletion in future clean up runs. Ideally, the operator will either keep namespace deletion disabled or upgrade the iproute package on the system to avoid these problems.


Configuration Handling

The handling of the config options will be cleaned up a bit; there’s so much ‘if that’ and ‘if this’ with config options too. Behavior needs to be properly encapsulated so that we don’t need to branch so much so often. A few examples examples are linked in the references [5] [6] [7] [8].


Data Model Impact




Security Impact

No impact is expected. We need to be careful when reviewing code that these changes do not introduce vulnerabilities in the agent.

Notifications Impact


Other End User Impact


Performance Impact


IPv6 Impact

We will take care to preserve all existing IPv6 functionality in Neutron. No changes or additions to the current IPv6 functionality are planned.

Other Deployer Impact


Developer Impact

Much of code in the file will be moved out to other files. This refactoring will introduce better software engineering patterns to allow the functionality to be extended, modified, and maintained more easily.

Developers who have become accustomed to the current implementation will likely not recognize the end result. However, they will be able to easily get reaquainted with the new code.

To avoid problems with rebasing and potential regressions while the heavy-lifting is being done, non-critical changes to the L3 agent should be avoided while this work is in progress. Mail will be sent to the openstack-dev ML to begin a freeze on non-critical changes and another one to end it. The freeze will only be needed during the initial more disruptive restructuring. As certain part stabilize, the freeze will be lifted. For example, once the VPN and FW services have been decoupled from the agent code – which will be the first step – development on those services can continue.

Community Impact

This change is part of the approved Neutron priorities for Kilo.

It supports at least the following efforts which may also be planned for Kilo.

  • Pluggable external networks blueprint (dynamic routing integration indirectly)
  • Enabling HA routers and DVR to work together.
  • Better integration of L3 services.
  • Spinning out advanced services


The alternative is to leave it like it is and to perform small bits of refactoring only when it is necessary for a particular new feature. This is not ideal since there are already a number of things that this refactoring needs to support. It will slow down the development of that work if this is delayed.

Writing a new agent and eventually deprecating the current one is another alternative? I’ve personally never had a very good experience with this approach. It seems to trade one set of known problems for another set of unknown problems. Regressions are all too common. I prefer to restructure in small reviewable pieces. This does not guarantee no regressions but it can uncover them earlier in the process and they are easier to pinpoint and fix.



Primary assignee:
Other contributors:
amuller jschwarz pcm yamahata

Work Items

I expect that some of the initial work items will need to be tackled in sequential order because of the high degree of coupling in the code. However, as things are decomposed and the coupling is reduced, other work items can be tackled in parallel.

For example, since the service agents are coupled with the L3 agent inheritence hierarchy, they will need to be moved out before a proper router abstraction is feasible.

  1. Functional Testing for the Agent

  2. Service Drivers

    • Start simple. This won’t be everything to everyone yet. It is not meant to full-blown pluggable service drivers.
    1. Metadata Proxy
    2. FWaaS
    3. VPNaaS
  3. Decomposition and modularization of DVR, HA, and legacy routers

    1. Create a proper abstraction of a router to replace RouterInfo
      • Can serve as an abstraction for other router implementations. Again, we’ll start simple to introduce the abstraction.
    2. Create the inheritence hierarchy.
      • This may be done in a few steps. Initially, the inheritence hierarchy may be thin with most of the implementation still in the base class. Future steps will move responsibilities to the sub-classes and evolve the interface.




In addition to the functional tests discussed below, effort will be made to use existing unit tests as necessary to be sure that existing coverage is retained and avoid regressions they were created to prevent. The end result may look like all of the old unit tests have been removed and new, better ones have been written in their place.

All new and restructured code will be covered with proper unit test coverage. It will be significantly easier to unit test with the new structure of the code. If it isn’t then we’re doing it wrong.

I don’t plan to make an effort to add missing unit test coverage before the code is restructured.

Tempest Tests

No new tempest tests are planned.

Functional Tests

Functional tests will be added from the L3 agent prior to any significant restructuring of the agent code. Assaf [9] will take the lead of this testing effort with help from John Schwarz [10] and all of the other assignees listed in this blueprint. This includes the addition of functional tests for the new DVR and HA [11] features.


API Tests

No new API tests are planned.

Documentation Impact


User Documentation


Developer Documentation

New API interfaces in the code will be documented with doc strings