  In NFV networks, service functions (SFs) can be deployed on virtual machines
(VMs) across multiple domains and then form a service function chain (MSFC) for
end-to-end network service provision. However, any software component in a
VM-based MSFC must experience software aging issue after a long period of
operation. This paper quantitatively investigates the capability of proactive
rejuvenation techniques in reducing the damage of software aging on a VM-based
MSFC. We develop a semi-Markov model to capture the behaviors of SFs, VMs and
virtual machine monitors (VMMs) from software aging to recovery under the
condition that failure times and recovery times follow general distributions.
We derive the formulas for calculating the steady-state availability and
reliability of the VM-based MSFC composed of multiple SFs running on VMs hosted
by VMMs. Sensitivity analysis is also conducted to identify potential
dependability bottlenecks.
