  Virtual Network Functions as a Service (VNFaaS) is currently under attentive
study by telecommunications and cloud stakeholders as a promising business and
technical direction consisting of providing network functions as a service on a
cloud (NFV Infrastructure), instead of delivering standalone network
appliances, in order to provide higher scalability and reduce maintenance
costs. However, the functioning of such NFVI hosting the VNFs is fundamental
for all the services and applications running on top of it, forcing to
guarantee a high availability level. Indeed the availability of an VNFaaS
relies on the failure rate of its single components, namely the servers, the
virtualization software, and the communication network. The proper assignment
of the virtual machines implementing network functions to NFVI servers and
their protection is essential to guarantee high availability. We model the High
Availability Virtual Network Function Placement (HA-VNFP) as the problem of
finding the best assignment of virtual machines to servers guaranteeing
protection by replication. We propose a probabilistic approach to measure the
real availability of a system and design both efficient and effective
algorithms that can be used by stakeholders for both online and offline
planning.
