  Network operators are facing significant challenges meeting the demand for
more bandwidth, agile infrastructures, innovative services, while keeping costs
low. Network Functions Virtualization (NFV) and Cloud Computing are emerging as
key trends of 5G network architectures, providing flexibility, fast
instantiation times, support of Commercial Off The Shelf hardware and
significant cost savings. NFV leverages Cloud Computing principles to move the
data-plane network functions from expensive, closed and proprietary hardware to
the so-called Virtual Network Functions (VNFs). In this paper we deal with the
management of virtual computing resources (Unikernels) for the execution of
VNFs. This functionality is performed by the Virtual Infrastructure Manager
(VIM) in the NFV MANagement and Orchestration (MANO) reference architecture. We
discuss the instantiation process of virtual resources and propose a generic
reference model, starting from the analysis of three open source VIMs, namely
OpenStack, Nomad and OpenVIM. We improve the aforementioned VIMs introducing
the support for special-purpose Unikernels and aiming at reducing the duration
of the instantiation process. We evaluate some performance aspects of the VIMs,
considering both stock and tuned versions. The VIM extensions and performance
evaluation tools are available under a liberal open source licence.
