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+.. This work is licensed under a Creative Commons Attribution 4.0 International License.
+.. http://creativecommons.org/licenses/by/4.0
+.. (c) Bin Hu
+
+
+Multiple Networking Backends
+----------------------------
+
+Description
+^^^^^^^^^^^
+
+Network Function Virtualization (NFV) brings the need of supporting multiple networking
+back-ends in virtualized infrastructure environments.
+
+First of all, a Service Providers' virtualized network infrastructure will consist of
+multiple SDN Controllers from different vendors for obvious business reasons.
+Those SDN Controllers may be managed within one cloud or multiple clouds.
+Jointly, those VIMs (e.g. OpenStack instances) and SDN Controllers need to work
+together in an interoperable framework to create NFV services in the Service
+Providers' virtualized network infrastructure. It is needed that one VIM (e.g. OpenStack
+instance) shall be able to support multiple SDN Controllers as back-end.
+
+Secondly, a Service Providers' virtualized network infrastructure will serve multiple,
+heterogeneous administrative domains, such as mobility domain, access networks,
+edge domain, core networks, WAN, enterprise domain, etc. The architecture of
+virtualized network infrastructure needs different types of SDN Controllers that are
+specialized and targeted for specific features and requirements of those different domains.
+The architectural design may also include global and local SDN Controllers.
+Importantly, multiple local SDN Controllers may be managed by one VIM (e.g.
+OpenStack instance).
+
+Furthermore, even within one administrative domain, NFV services could also be quite diversified.
+Specialized NFV services require specialized and dedicated SDN Controllers. Thus a Service
+Provider needs to use multiple APIs and back-ends simultaneously in order to provide
+users with diversified services at the same time. At the same time, for a particular NFV service,
+the new networking APIs need to be agnostic of the back-ends.
+
+
+
+Requirements
+^^^^^^^^^^^^
+
+Based on the use cases described above, we derive the following
+requirements.
+
+It is expected that in NFV networking service domain:
+
+* One OpenStack instance shall support multiple SDN Controllers simultaneously
+
+* New networking API shall be integrated flexibly and quickly
+
+* New NFV Networking APIs shall be agnostic of back-ends
+
+* Interoperability is needed among multi-vendor SDN Controllers at back-end
+
+
+
+Current Implementation
+^^^^^^^^^^^^^^^^^^^^^^
+
+In the current implementation of OpenStack networking, SDN controllers are
+hooked up to Neutron by means of dedicated plugins.  A plugin translates
+requests coming in through the Neutron northbound API, e.g. the creation of a
+new network, into the appropriate northbound API calls of the corresponding SDN
+controller.
+
+There are multiple different plugin mechanisms currently available in Neutron,
+each targeting a different purpose. In general, there are `core plugins`,
+covering basic networking functionality and `service plugins`, providing layer 3
+connectivity and advanced networking services such as FWaaS or LBaaS.
+
+
+
+Core and ML2 Plugins
+''''''''''''''''''''
+
+The Neutron core plugins cover basic Neutron functionality, such as creating
+networks and ports. Every core plugin implements the functionality needed to
+cover the full range of the Neutron core API. A special instance of a core
+plugin is the ML2 core plugin, which in turn allows for using sub-drivers -
+separated again into type drivers (VLAN, VxLAN, GRE) or mechanism drivers (OVS,
+OpenDaylight, etc.). This allows to using dedicated sub-drivers for dedicated
+functionality.
+
+In practice, different SDN controllers use both plugin mechanisms to integrate
+with Neutron. For instance OpenDaylight uses a ML2 mechanism plugin driver
+whereas OpenContrail integrated by means of a full core plugin.
+
+In its current implementation, only one Neutron core plugin can be active at any
+given time. This means that if a SDN controller utilizes a dedicated core
+plugin, no other SDN controller can be used at the same time for the same type
+of service.
+
+In contrast, the ML2 plugin allows for using multiple mechanism drivers
+simultaneously. In principle, this enables a parallel deployment of multiple SDN
+controllers if and only if all SDN controllers integrate through a ML2 mechanism
+driver.
+
+
+
+Neutron Service Plugins
+'''''''''''''''''''''''
+
+Neutron service plugins target L3 services and advanced networking services,
+such as BGPVPN or LBaaS. Typically, a service itself provides a driver plugin
+mechanism which needs to be implemented for every SDN controller. As the
+architecture of the driver mechanism is up to the community developing the
+service plugin, it needs to be analyzed for every driver plugin mechanism
+individually if and how multiple back-ends are supported.
+
+
+
+Gaps in the current solution
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Given the use case description and the current implementation of OpenStack
+Neutron, we identify the following gaps:
+
+
+* **[MB-GAP1] Limited support for multiple back-ends**
+
+  As pointed out above, the Neutron core plugin mechanism only allows for one
+  active plugin at a time. The ML2 plugin allows for running multiple mechanism
+  drivers in parallel, however, successful inter-working strongly depends on the
+  individual driver.
+
+  Moreover, the ML2 plugin and its API is - by design - very layer 2 focused. For
+  NFV networking use cases beyond layer 2, for instance L3VPNs, a more flexible
+  API is required.
+
+
+Conclusion
+^^^^^^^^^^
+
+We conclude that a complementary method of integrating multiple SDN controllers
+into a single OpenStack deployment is needed to fulfill the needs of operators.