rst packet. The inline services may (optionally) notify the OF controller that cer- tain content or metrics have been identified via the StEERING Service Protocol, shown as the grey dotted lines in Figure 5.5. For instance, once DPI has recognized or resolved a flow, it can send a notification to the OF controller. The data plane (forwarding) can be easily configured and scale as the number of subscriber/application combination grows. The controller programs switches with the rules on how to forward each packet. For- warding decisions are made based on Layer 2 to Layer 4 contents of packets as well as the ingress port. The key challenge to achieve scala- bility is to avoid exponential growth (rule explosion) of the forwarding rules installed in each switch. We make three design choices to reduce the amount of state on each switch: defining port types to indicate directions, using multiple tables to decompose multidimensional poli- cies, and introducing a new metadata type to encode service paths. 5 SDN and NFV in 5G 5.2.1.1 Represent Directions with Port Types We define two types of ports on perimeter switches: node ports and transit ports. Node ports are connected to services and gateway nodes (BNG, GGSN, routers). Transit ports are connected to other perimeter switches or to inner network switches. Figure 5.6 shows an example of a service delivery network based on the StEERING architecture. Switches OF1, OF2, and OF3 are perime- ter switches. Switch SW1 is an inner switch. The black/grey ports on the switches are node ports and the white ports are transit ports. Incoming traffic, either coming in from a gateway node or coming back from a service, always enters the service delivery network via a perimeter switch and through a node port. Packets coming in through node ports are steered toward the next node (service or gateway) in their assigned service paths. Packets arriving on transit ports are sim- ply forwarded using their destination MAC address. All packets traversing the steering network are considered to 