'Allocate resource for a new tenant network segment.
:param session: database session
:returns: segment dictionary using keys defined above
Called inside transaction context on session to allocate a new
tenant network, typically from a type-specific resource
pool. If successful, return a segment dictionary describing
the segment. If tenant network segment cannot be allocated
(i.e. tenant networks not supported or resource pool is
exhausted), return None.'

@abc.abstractmethod
def allocate_tenant_segment(self, session):

    pass
