  Ideally, by enabling multi-tenancy, network virtualization allows to improve
resource utilization, while providing performance isolation: although the
underlying resources are shared, the virtual network appears as a dedicated
network to the tenant. However, providing such an illusion is challenging in
practice, and over the last years, many expedient approaches have been proposed
to provide performance isolation in virtual networks, by enforcing bandwidth
reservations. We in this paper study another source for overheads and
unpredictable performance in virtual networks: the hypervisor.
  The hypervisor is a critical component in multi-tenant environments, but its
overhead and influence on performance are hardly understood today. In
particular, we focus on OpenFlow-based virtualized Software Defined Networks
(vSDNs). Network virtualization is considered a killer application for SDNs: a
vSDN allows each tenant to flexibly manage its network from a logically
centralized perspective, via a simple API. For the purpose of our study, we
developed a new benchmarking tool for OpenFlow control and data planes,
enabling high and consistent OpenFlow message rates. Using our tool, we
identify and measure controllable and uncontrollable effects on performance and
overhead, including the hypervisor technology, the number of tenants as well as
the tenant type, as well as the type of OpenFlow messages.
