VIF port config versioned objects and driver plugin library

Define a standalone os-vif python library, inspired by os-brick, to provide a versioned object model for data passed from neutron to nova for VIF port binding, and an API to allow vendors to provide custom plug/unplug actions for execution by Nova.

Problem description

When plugging VIFs into VM instances there is communication between Nova and Neutron to obtain a dict of port binding metadata. Nova passes this along to the virt drivers which have a set of classes for dealing with different VIF types. In the libvirt case, each class has three methods, one for building the libvirt XML config, one for performing host OS config tasks related to plugging a VIF and one for performing host OS config tasks related to unplugging a VIF.

Currently, whenever a new Neutron mechanism driver is created, this results in the definition of a new VIF type, and the addition of a new VIF class to the libvirt driver to support it. Due to the wide variety of vendors, there is a potentially limitless number of Neutron mechanisms that need to be dealt with over time. Conversely the number of different libvirt XML configurations is quite small and well defined. There are sometimes new libvirt XML configs defined, as QEMU gains new network backends, but this is fairly rare. Out of 15 different VIF types supported by libvirt’s VIF driver today, there are only 5 distinct libvirt XML configurations required. These are illustrated in

The problem with this architecture is that the Nova libvirt maintainers have task of maintaining the plug/unplug code in the VIF drivers, which is really code that is defined by the needs of the Neutron mechanism. This prevents Neutron project / vendors from adding new VIF types without having a lock-step change in Nova.

A second related problem, is that the format of the data passed between Nova and Neutron for the VIF port binding is fairly loosely defined. There is no versioning of the information passed between them and no agreed formal specification of what the different fields mean. This data is used both to generate the libvirt XML config and to control the logic of the plug/unplug actions.

Use Cases

The overall use case is to facilitate the creation of new Neutron mechanisms by removing Nova as a bottleneck for work. New features can be implemented entirely in the Neutron codebase (or mechanism specific codebase) with no need to add/change code in Nova, in the common case.

Proposed change

Inspired by the os-brick library effort started by the Cinder project, the proposal involves creation of a new library module that will be jointly developed by the Neutron & Nova teams, for consumption by both projects.

This proposal is describing an architecture with the following high level characteristics & split of responsibilities

  • Definition of VIF types and associated config metadata.

    • Owned jointly by Nova and Neutron core reviewer teams
    • Code shared in os-vif library
    • Ensures core teams have 100% control over data on the REST API
  • Setup of compute host OS networking stack

    • Owned by Neutron mechanism vendor team
    • Code distributed by mechanism vendor
    • Allows vendors to innovate without bottleneck on Nova developers in common case.
    • In the uncommon, event a new VIF type was required, this would still require os-vif modification with Nova & Neutron core team signoff.
  • Configuration of guest virtual machine VIFs ie libvirt XML

    • Owned by Nova virt driver team
    • Code distributed as part of Nova virt / VIF driver
    • Ensures hypervisor driver retains full control over how the guest instances are configured

Note that while the description below frequently refers to the Nova libvirt driver, this proposal is not considered libvirt specific. The same concepts and requirements for VIF type support exist in all the other virt drivers. They merely support far fewer different VIF types than libvirt, so the problems are not so immediately obvious in them.

The library will make use of the oslo.versionedobjects module in order to formally define a set of objects to describe the VIF port binding data. The data in this objects will be serialized into JSON, for transmission between Neutron and Nova, just as is done with the current dicts used today. The difference is that by using oslo.versionedobjects, we gain a formal specification and the ability to extend and modify the objects over time in a manner that is more future proof. One can imagine a base object

from oslo_versionedobjects import base

class VIFConfig(base.VersionedObject)
    # Common stuff for all VIFs
    fields = {
        # VIF port identifier
        id:  UUIDField()

        # Various common fields see current
        # class and related ones

        # Name of the class used for VIF (un)plugging actions
        plug: StringField()

        # Port profile metadata  - needed for network modes
        # like OVS, VEPA, etc
        profile: ObjectField("VIFProfile")

This base object defines the fields that are common to all the different VIF port binding types. There are a number of these attributes, currently detailed in the VIF class in, or the equiv in Neutron.

One addition here is a ‘plug’ field which will be the name of a class that will be used to perform the vendor specific plug/unplug work on the host OS. The supported values for the ‘plug’ field will be determined by Nova via a stevedore based registration mechanism. Nova can pass this info across to Neutron, so that mechanisms know what plugins have been installed on the Nova compute node too. Tagging the plugin class with a version will also be required to enable upgrades where the Neutron mechanism versions is potentially newer than the nova installed plugin.

This ‘plug’ field is what de-couples the VIF types from the vendor specific work, and will thus allow the number of VIFConfig classes to remain at a fairly small finite size, while still allowing arbitary number of Neutron mechanisms to be implemented. As an example, from the current list of VIF types shown at:

We can see that IVS, IOVISOR, MIDONET and VROUTER all use the same libvirt type=ethernet configuration, but different plug scripts. Similarly there is significant overlap between VIFs that use type=bridge, but with different plug scripts.

The various VIFConfig subclasses will be created, based on the different bits of information that are currently passed around. NB, this is not covering all the current VIF_TYPE_XXX variants, as a number of them have essentially identical config parameter requirements, and only differ in the plug/unplug actions, hence the point previously about the ‘plug’ class name. All existing VIF types will be considered legacy. These various config classes will define a completely new set of modern VIF types. In many cases they will closely resemble the existing VIF types, but the key difference is in the data serialization format which will be using oslo.versionedobject serialization instead of dicts. By defining a completely new set of VIF types, we make it easy for Nova to negotiate use of the new types with Neutron. When calling Neutron, Nova will indicate what VIF types it is capable of supporting, and thus Neutron can determine whether it is able to use the new object based VIF types or the legacy anonymous dict based types.

The following dependant spec describes a mechanism for communicating the list of supported VIF types to Neutron when Nova creates a VIF port.

What is described in that spec will need some further improvements. Instead of just a list of VIF types, it will need to be a list of VIF types and their versions. This will allow Neutron to back-level the VIF object data to an older version in the event that Neutron is running a newer version of the os-vif library than is installed on the Nova compute host. Second, in addition to the list of VIF types, Nova will also need to provide a list of installed plugins along with their versions.

So approximately the following set of objects would be defined to represent the new VIF types. It is expected that the result of the ‘obj_name()’ API call (defined by oslo VersionedObject base class) will be used as the VIF type name. This gives clear namespace separation from legacy VIF type names.

class VIFConfigBridge(VIFConfig):
    fields = {
        # Name of the host TAP device used as the VIF
        devname: StringField(nullable=True)

        # Name of the bridge device to attach VIF to
        bridgename: StringField()

class VIFConfigEthernet(VIFConfig):
    fields = {
        # Name of the host TAP device used as the VIF
        devname: StringField()

class VIFConfigDirect(VIFConfig):
    fields = {
        # Source device NIC name on host (eg eth0)
        devname: StringField()
        # An enum of 'vepa', 'passthrough', or 'bridge'
        mode: DirectModeField()

class VIFConfigVHostUser(VIFConfig):
    fields = {
        # UNIX socket path
        path: StringField()

        # Access permission mode
        mode: StringField()

class VIFConfigHostDevice(VIFConfig):
    fields = {
        # Host device PCI address
        devaddr: PCIAddressField()

        # VLAN number
        vlan: IntegerField()

NB, the attributes listed in these classes above are not yet totally comprehensive. At time of implementation, there will be more thorough analysis of current VIF code to ensure that all required attributes are covered.

This list is based on the information identified in this wiki page

Some of these will be applicable to other hypervisors too, but there may be a few more vmware/hypervisor/xenapi specific config subclasses needed too. This spec does not attempt to enumerate what those will be yet, but they will be similarly simple and finite set.

Those looking closely will have see reference to a “VIFProfile” object in the “VIFConfig” class shown earlier. This object corresponds to the data that can be provided in the <portprofile>...</portprofile> XML block. This is required data when a VIF is connected to OpenVSwitch, or when using one of the two VEPA modes. This could have been provided inline in the the VIFConfig subclasses, but there are a few cases where the same data is needed by different VIF types, so breaking it out into a separate object allows better reuse, without increasing the number of VIF types.

class VIFProfile(base.VersionedObject):

class VIFProfile8021QBG(VIFProfile):
     fields = {
       managerid: IntegerField(),
       typeid: IntegerField()
       typeidversion: IntegerField()
       instanceid: UUIDField()

class VIFProfile8021QBH(VIFProfile):
     fields = {
       profileid: StringField()

class VIFProfileOpenVSwitch(VIFProfile):
     fields = {
       interfaceid: UUIDField()
       profileid: StringField()

Finally, as alluded to in an earlier paragraph, the library will also need to define an interface for enabling the plug / unplug actions to be performed. This is a quite straightforward abstract python class

class VIFPlug(object):

    VERSION = "1.0"

    def plug(self, config):
      raise NotImpementedError()

    def unplug(self, config):
      raise NotImpementedError()

The ‘config’ parameter passed in here will be an instance of the VIFConfig versioned object defined above.

There will be at least one subclass of this VIFPlug class provided by each Neutron vendor mechanism. These subclass implementations do not need to be part of the os-vif library itself. The mechanism vendors would be expected to distribute them independently, so decomposition of the neutron development is maintained. It is expected the vendors will provide a separate VIFPlug impl for each hypervisor they need to be able to integrate with, so info about the Nova hypervisor must be provided to Neutron when Nova requests creation of a VIF port. The VIFPlug classes must be registered with Nova via the stevedore mechanism, so that Nova can identify the list of implementations it has available, and thus validate requests from Neutron to use a particular plugin. It also allows Nova to tell Neutron which plugins are available for use. The plugins will be versioned too, so that it is clear to Neutron which version of the plugin logic will be executed by Nova.

The vendors would not be permitted to define new VIFConfig sub-classes, these would remain under control of the os-vif library maintainers (ie Neutron and Nova teams), as any additions to data passed over the REST API must be reviewed and approved by project maintainers. Thus proposals for new VIFConfig classes would be submitted to the os-vif repository where the will be reviewed jointly by the Nova & Neutron representatives working on that library. It is expected that this will be a fairly rare requirement, since most new mechanism can be implemented using one of the many existing VIFConfigs.

So when a vendor wishes to create a new mechanism, they first decide which VIFConfig implementation(s) they need to target, and populate that with the required information about their VIF. This information is sufficient for the Nova hypervisor driver to config the guest virtual machine. When instantiating the VIFConfig impl, the Neutron vendor will set the ‘plug’ attribute to refer to the name of the VIFPlug subclass they have implemented with their vendor specific logic. The vendor VIFPlug subclasses must of course be installed on the Nova compute nodes, so Nova can load them.

When Nova asks Neutron to create the VIF, neutron returns the serialized VIFConfig class, which Nova loads. Nova compute manager passes this down to the virt driver implementation, which instantiates the class defined by the ‘plug’ attribute. It will then invoke either the ‘plug’ or ‘unplug’ method depending on whether it is attaching or detaching a VIF to the guest instance. The hypervisor driver will then configure the guest virtual machine using the data stored in the VIFConfig class.

When a new Nova talks to an old Neutron, it will obviously be receiving the port binding data in the existing dict format. Nova will have to have some compatibility code to be able to support comsumption of the data in this format. Nova would likely convert the dict on the fly to the new object model. The existing libvirt driver VIF plug/unplug methods would also need to be turned into VIFPlug subclasses. This way new Nova will be able to deal with all pre-existing VIF types that old Neutron knows about, with no loss in functionality.

When an old Nova talks to a new Neutron, Neutron will have to return the data in the existing legacy port binding format. For this to work, there needs to be a negotiation between Nova and Neutron to opt-in to use of the new VIFConfig object model. With an explicit opt-in required, when an old Nova talks to new Neutron, Neutron will know to return data in the legacy format that Nova can still understand. The obvious implication of this is that any newly developed Neutron mechanisms that rely on the new VIFCOnfig object model exclusively, will not work with legacy Nova deployments. This is not considered to be a significant problem, as the mis-match in Neutron/Nova versions is only a temporary problem as a cloud undergoes a staged update from Kilo to Liberty

To aid in understanding how this changes from current design, it is helpful to compare the relationships between the objects. Currently there is mostly a 1:1 mapping between Neutron mechanisms, vif types, and virt driver plugins. Thus each new Neutron mechanism has typically needed a new VIF type and virt driver plugin.

In this new design, there will be the following relationships

  • VIF type <-> VIFConfig class - 1:1 - VIFConfig classes are direct representation of each VIF type - a VIF type is simply the name of the class used to represent the data.
  • Neutron mechanism <-> VIF type - M:N - A single mechanism can use one or more VIF types, a particular choice made at runtime based on usage scenario. Multiple mechanisms will be able to use the same VIF type
  • VIF type <-> VIF plugins - 1:M - a single VIF type can be used with multiple plugins. ie many mechanisms will use the same VIF type, but each supply their own plugin implementation for host OS setup

The split between VIF plugins and VIF types is key to the goal of limiting the number of new VIF types that are created over time.


  1. Do nothing. Continue with the current approach where every new Neutron mechanism requires a change to Nova hypervisor VIF driver to support its vendor specific plug/unplug actions. This will make no one happy.

  2. Return to the previous approach, where Nova allows loading of out of tree VIF driver plugins for libvirt. This is undesirable for a number of reasons.

    The task of configuring a libvirt guest consists of two halves commonly referred to as backend configuration (ie the host) and frontend configuration (ie what the guest sees). The frontend config is something that the libvirt driver needs to retain direct control over, in order to support various features that are common to all VIFs regardless of backend config.

    In addition the libvirt driver has a set of classes for representing the libvirt XML config of a guest, which need to be capable of representing any VIF config for the guest. These are considered part of the libvirt internal implementation and not a stable API.

    Thirdly, the libvirt VIF driver plugin API has changed in the past and may change again in the future, and the data passed into it is an ill-defined dict of values from the port binding.

    For these reasons there is a strong desire to not hand off the entire implementation of the current libvirt VIF driver class to an external 3rd party.

    That all said, this spec does in fact take things back to something that is pretty similar to this previous approach. The key differences and benefits of this spec, are that it defines a set of versioned objects to hold the data that is passed to the 3rd party VIFPlug implementation. The external VIFPlug implementation is only being responsible for the host OS setup tasks - ie the plug/unplug actions. The libvirt driver retains control over guest configuration The VIFPlug driver is isolated from the internal impl and API design of the libvirt hypervisor driver. The commonality is that the Neutron vendor has the ability to retain control of their plug/unplug tasks without Nova getting in the way.

  3. Keep the current VIF binding approach, but include the name of an executable program (script) that Nova will invoke to perform the plug/unplug actions.

    This is approximately the same as the proposal in this spec, it is just substituting in-process execution of python code, for out of process execution of a (shell) script. In the case of scripts, the data from the VIF port bindings must be provided to the script, and the proposal was to use environment variables. This is moderately ok if the data is all scalar, but if there is as need to provide non-scalar structured data like dicts/lists, then the environment variable approach is very painful to work with.

    The VIF script approach also involves creation of some formal versioned objects for representing port binding data, but those objects live inside Nova. Since Neutron has the same need to represent the VIF port binding data, it is considered better if we can have an external python module which defines the versioned objects to represent the port binding data, that can be shared between both Nova and Neutron

    It is believed that by defining a formal set of versioned objects to represent the VIF port binding data, and a python abstract class for the plug/unplug actions, we achieve a strict, clean and easily extensible interface for the boudnary between Nova and Neutron, avoiding some of the problems inherant in serializing the data via environment variables. ie the VIFPlug subclasses will stil get to access the well defined VIFConfig class attributes, instead of having to parse environment variables.

  4. As per this spec, but keep all the VIFConfig classes in Nova instead of creating a separate os-vif library. The main downside with this is that Neutron will ultimately need to create its own copy of the VIFConfig classes, and there will need to be an agreed serialization format between Nova and Neutron for the VIF port binding metadata passed over the REST API. By having the VIFConfig classes in a library that can be used by both Nova and Neutron directly, we ensure both apps have a unified object model and can leverage the standard oslo.versionedobject serialization format. This brings Neutron/Nova a well defined REST API data format this the data passed between them.

  5. Move responsibility for VIF plug/unplug to Neutron. This would require that Neutron provide an agent to run on every compute node that takes care of the plug/unplug actions. This agent would have to have a plugin API so that each Neutron mechanism can provide its own logic for the plug/nuplug actions. In addition the agent would have to deal with staged upgrades where an old agent works with new Neutron or a new agent works with old Neutron. There would still need to be work done to formalize the VIF config data passed between Neutron and Nova for the purpose of configuring the guest instance. So this alternative is ultimately pretty similar to what is described in this spec. The current proposal can simply be thought of as providing this architecture, but with the agent actually built-in to Nova. Given the current impl of Neutron & Nova, leveraging Nova as the “agent” on the compute nodes is lower effort approach with no strong downsides.

Data model impact

There is no change to the database data model.

REST API impact

This work requires the aforementioned spec to allow Nova to pass details of its supported VIF types to Neutron:

For existing “legacy” VIF types, the data format passed back by Neutron will not change.

For the new “modern” VIF types, the data format passed back by Neutron will use the oslo.versionedobjects serialization format, instead of just serializing a plain python dict. In other words, the data will be the result of the following API call


where cfg is the VIFConfig versioned object. This JSON data is thus formally specified and versioned, improving ability to evolve this in future releases.

In terms of backwards compatibility there are the following scenarios to consider

  • Old Neutron (Kilo), New Nova (Liberty)

    Nova adds extra info to the request telling Neutron what VIF types and plugins are supported. Neutron doesn’t know about this so ignores it, and returns one of the legacy VIF types. Nova libvirt driver transforms this legacy VIF type into a modern VIF type, using one of its a built-in back-compat plugins. So there should be no loss in functionality compared to old Nova

  • New Neutron (Liberty), Old Nova (Kilo)

    Nova does not add any info to the request telling Neutron what VIF types are supported. Neutron assumes that Nova only supports the legacy VIF types and so returns data in that format. Neutron does not attempt to use the modern VIF types at all.

  • New Neutron (Liberty), New Nova (Liberty)

    Nova adds extra info to the request telling Neutron what VIF types and plugins are supported. The neutron mechanism looks at this and decides which VIF type + plugin it wishes to use for the port. Neutron passes back a serialized VIFConfig object instance. Nova libvirt directly uses its modern code path for VIF type handling

  • Even-newer Neutron (Mxxxxx), New-ish Nova (Liberty)

    Nova adds extra info to the request telling Neutron what VIF types and plugins are supported. Neutron sees that Nova only supports VIFConfigBridge version 1.0, but it has version 1.3. Neutron thus uses obj_make_compatible() to backlevel the object to version 1.0 before returning the VIF data to Nova.

  • New-ish Neutron (Liberty), Even-newer Nova (Mxxxx)

    Nova adds extra info to the request telling Neutron what VIF types and plugins are supported. Neutron only has version 1.0 but Nova supports version 1.3. Nova can trivially handle version 1.0, so Neutron can just return data in version 1.0 format and Nova just loads it and runs.

Security impact

The external VIFPlug classes provided by vendors will be able to run arbitrary code on the compute nodes. This is little different in security risk than the current situation where the libvirt VIF driver plug/unplug method implementations run a fairly arbitrary set of commands on the compute host. One difference though is that the Nova core team will no longer be responsible for reviewing that code, as it will be maintained exclusively by the Neutron mechanism vendor.

While it is obviously possible to vendors to add malicious code to their plugin. This isn’t a complete free for all though - the cloud admin must have taken explicit action to install this plugin on the compute node and have it registered appropriately via stevedore. So this does not allow arbitrary code execution by Neutron.

Notifications impact


Other end user impact


Performance Impact


Other deployer impact

When deploying new Neutron mechanisms, they will include a python module which must be deployed on each compute host. This provides the host OS plug/unplug logic that will be run when adding VIFs to a guest.

In other words, while currently a user deploying a mechanism would do

pip install neutron-mech-wizzbangnet

on the networking hosts, in the new system they must also run

pip install nova-vif-plugin-wizzbangnet

on any compute nodes that wish to integrate with this mechanism.

It is anticipated that the various vendor tools for deploying openstack will be able to automate this extra requirement, so cloud admins will not be appreciably impacted by this.

Developer impact

When QEMU/libvirt (or another hypervisor) invents a new way of configuring virtual machine networking, it may be neccessary to define a new versioned object in the os-vif library that is shared between Neutron and Nova. This will involve defining a subclass of VIFConfig, and then implementing the logic in the Nova libvirt driver to handle this new configuration type. Based on historical frequency of such additions in QEMU, it is expected that this will be a rare occurrance.

When a vendor wishes to implement a new Neutron mechanism, they will have to provide an implementation of the VIFPlug class whose abstract interface is defined in the os-vif library. This vendor specific implementation will not need to be included in the os-vif library itself - it can be distributed and deployed by the vendor themselves. This frees the vendor from having to do a lock-step update to Nova to support their product.



Primary assignee:

Other contributors:

Daniel Berrange <> irc:danpb Brent Eagles <> irc: beagles Andreas Scheuring Maxime Leroy Jay Pipies irc: jaypipes

Work Items

  1. Create a new os-vif python module in openstack and/or stackforge
  2. Implement the VIFConfig abstract base class as a versioned object using oslo.versionedobjects.
  3. Agree on and define the minimal set of VIF configurations that need to be supported. This is approximately equal to the number of different libvirt XML configs, plus a few for other virt hypervisors
  4. Create VIFConfig subclasses for each of the configs identified in step 3.
  5. Define the VIFPlug abstract base class for Neutron mechanism vendors to implement
  6. Extend Neutron such that it is able to ask mechansisms to return VIF port data in either the legacy dict format or as a VIFConfig object instance
  7. Extend Nova/Neutron REST interface so that Nova is able to request use of the VIFConfig data format
  8. Add code to Nova to convert the legacy dict format into the new style VIFConfig object format, for back compat with old Neutron
  9. Convert the Neutron mechanisms to be able to use the new VIFConfig object model
  10. Profit


The key dependency is to have collaboration between the Nova and Neutron teams in setting up the new os-vif python project, and defining the VIFConfig object model and VIFPlug interface.

There is also a dependancy in agreeing how to extend the REST API in Neutron to allow Nova to request use of the new data format. This is discussed in more detail in:

Though some aspects of that might need updating to take account of the proposals in this spec

Once those are done, the Nova and Neutron teams can progress on their respective work items independently.


The current gate CI system includes cover for some of the Neutron mechanisms. Once both Neutron and Nova support the new design, the current CI system will automatically start to test its operation.

For Neutron mechanisms that are not covered by current CI, it is expected that the respective vendors take on the task of testing their own implementations, as is currently the case for 3rd party CI.

Documentation Impact

The primary documentation impact is not user facing. The docs required will all be developer facing, so can be done as simple docs inside the respective python projects.

There will be some specific release notes required to advise cloud admins of considerations during upgrade. In particular when upgrading Nova it will be desired to deploy one or more of the Nova VIF plugins to match the Neutron mechanism(s) that they are currently using. If they fail to deploy the plugin, then the Nova/Neutron negotiation should ensure that Neutron continues to use the legacy VIF type, instead of switching to the modern VIF type.


The proposal to add a negotiation between Neuton and Nova for vif port binding types. This is a pre-requisite for this spec

The alternative proposal to introduce a VIF script to the existing VIF port binding data. This spec obsoletes that.

The alternative proposal to completely outsource hyervisor VIF driver plugins to 3rd parties once again. This spec obsoletes that.

Basic impl of library suggested by Jay Pipes

Variant of Jay’s design, which more closely matches what is described in this spec


Release Name Description
Liberty Introduced