Cloned from launchpad blueprint https://blueprints.launchpad.net/neutron/+spec/extensions-in-ml2.

Description:

In the current ML2 plugin implementation, only the extensions defined in the
Ml2Plugin class itself are available, and there is no way for core resources to
be extended with attributes needed by specific mechanism drivers. When such
extensions are needed, whether for new general purpose features that might be
incorporated in future core API versions, or to better support specific
networking technologies, the only current option is to implement a new
monolithic plugin

Specification URL (additional information):

http://git.openstack.org/cgit/openstack/neutron-specs/tree/specs/juno/neutron-ml2-mechanismdriver-extensions.rst