## 3 Reference Service Scenarios  
In this section, we survey two main reference service scenarios for virtualized networks made possible by dynamic VNF composition and orchestration (see Figure 1). The first scenario is enabled by the deployment of VNFs that provide network element (NE) functions, _i.e._, VNF-NEs. Examples of NEs might be either switching elements (e.g., routers, broadband remote access server), mobile network nodes (HLR/HSS, SGSN, GGSN/PDN-GW, base stations), or signaling control systems (e.g., session border controllers) [12]. Different functions of the same NE can be also partitioned and deployed as different VNFs operated in different network locations instead as one single comprehensive VNF. For instance, a session border controller can be split in the following functions deployed as VNFs, _i.e._, session terminations executed at the edge of the network, admission control executed in the core network, and statistics and billing data collection executed at a data center [2]. Figure 1: Scenarios for dynamic composition of Virtual Network Functions. In general terms, the sequence of deployed VNFs formally represents a virtual network (VN) established to deliver different kinds of network infrastructure services (e.g., IMS, 4G, IP/MPLS) and loosely coupled with the exact physical location of constituent VNFs. On the one hand, the deployment of VNF-NE chains allows for NSPs to set-up/upgrade the network infrastructure while taking advantage of the deployment of network functions as virtual machines. On the other hand, the NSPs can enrich their service offerings through the delivery of virtualized network infrastructure services to third-parties, _i.e._, \"VN service\". In a scenario where network functions can be dynamically discovered, negotiated and elastically composed as services, application service providers may lease VNF-NE chains with given communication capabilities from different NSPs and compose them to operate an end-to-end virtual service infrastructure to offer value-added application services to users (e.g., delay-optimized infrastructure for high-definition video applications [6]). Accordingly, a request for \"VN service\" set-up can be internally issued by a Network Management System (NMS) or an Operations Support System (OSS) on behalf of a network planning function of a NSP to establish/update the network infrastructure. Furthermore, a request for a virtual service infrastructure deployment, such as a next generation service overlay network (NGSON), can be issued by an application service provider. The second scenario derives from the deployment of VNFs providing middlebox (MB) functions, _i.e._, VNF-MBs. Example of MBs are: firewall, network address translation, WAN optimization controller (WOC), deep packet inspection (DPI), intrusion detection systems, load balancers, multimedia transcoders, virus scanning. Elastic deployments of VNF-MBs allow for differentiated data processing since service data flows can traverse only the needed VNFs, while skipping the unnecessary ones. For instance, in the case of a long-lived multimedia data flow it could be beneficial to avoid a DPI appliance in order to confine delays and save processing resources, while it could be beneficial including a WOC function so that traffic would be routed over links with proper level of delay and jitter guarantees. Accordingly, a service delivery path need to be established to serve an application data flow which includes specified network processing functions to be traversed for addressing given application requirements. The capability to dynamically establish chains of VNF-MBs allows NSPs enriching their Quality of Service (QoS) offerings through the delivery of paths with extended QoS guarantees that do not only address delay or throughput requirements but also availability, reliability and security requirements demanded by the application data flows, _i.e._, \"flow service with custom data treatment\".