rvices and applications interruption time should be less than 50 ms. However, requirements may be stricter for certain mission-critical services (e.g. control of driverless cars or drones via a mobile network). Since DPI influences the performance of services, a certain number of DPI perfor- mance metrics need to be defined [23]. Such metrics should comply with national (or regional) laws, policies, and regulations. The DPI performance metrics are based on the defined DPI functions and measured at defined measurement points. The DPI perfor- mance model is shown in Figure 3.12. DPI node (e.g. router) Control plane Management plane DPI policy management DPI policy decision functions functions Configuration of Policy control DPI policy rules Alerts User plane DPI node functions Packet forwarding Local policy functions decision base Forwarding Update information base (FIB) DPI policy information base (PIB) Results Policy control Conditions Conditions & actions Determining DPI scan analyzer execution next node function function function function User packets DPI - Deep packet inspection Control information Figure 3.12 DPI performance model. 3 QoS in NGN and Future Networks DPI performance is defined as a matrix, which includes three DPI functions: application identification, PIB (policy information base) maintenance, and system status. Performance is measured with respect to four metrics: speed, accuracy, depend- ability, and resources [24]. Certain DPI performance metrics are defined for the user plane traffic, and others are defined for the control or management plane traffic, as given in Table 3.4. One of the key requirements of DPI is identification of the application, with the aim of ensuring QoS and QoE (when applicable). Such typical applications over public Internet access are HTTP (i.e. Web), email protocols, peer-to-peer applications (e.g. BitTorrent), video streaming (e.g. "tube" video sharing sites such as YouTube), etc. However, one should note that network neutrality applied to Internet access service do