  Value-added Services (VASs) (e.g. dynamic site acceleration, media
management) play a critical role in Content Delivery Networks (CDNs). Network
Functions Virtualization (NFV) enables the agile provisioning of VASs. In NFV
settings, VASs are provisioned as ordered sets of Virtual Network Functions
(VNFs), forming VNF-Forwarding Graphs (VNF-FG) which are deployed in the CDN
infrastructure. The CDN VAS VNF-FGs have a specific characteristic: they have
one end-point (corresponding to the content server) that is unknown, prior to
their placement. The proposals for CDN VAS VNF-FG placement, so far, have only
considered offline placement, where the VNF-FGs are placed before end-user
traffic steers into the network. However, in concrete cases, a change in
service usage patterns might occur, a situation that could require a VNF-FG
placement in an online manner. This paper tackles the problem of online VNF-FG
placement for VASs in CDNs, taking into account the eventual reuses and
migrations of already-deployed VNFs. A cost model is considered, including
multiple costs; i.e. new VNF instantiations, migration, hosting and routing
costs. The objective is to optimally place the VNF-FGs such that total
reconfiguration costs are minimized while QoS is satisfied. An Integer Linear
Programming (ILP) formulation is provided and evaluated in a small-scale
scenario.
