# Mobility-Aware Hybrid Flow Rule Cache Scheme in Software-Defined Access Networks
## 1 Introduction  
Software-defined networking (SDN) decouples the control plane from the forwarding plane via a programmable interface between two planes, such as OpenFlow [1]. The separated SDN architecture supports a global network view and flow-based fine-granular traffic control. Based on the separated architecture, SDN has been researched with respect to enhancement of network operation efficiency. For example, SDN enables unified control, which is supported by the device, edge, and orchestration controllers [2]. In addition, SDN allows heterogeneous layered networks in terms of radio nodes as well as operators to be optimized by the SDN orchestrator [3]. Moreover, in combination with mobile edge computing (MEC), SDN has been applied in 5G networks to design dynamic, manageable, and cost-effective networks. For instance, SDN can perform the decision making for MEC server selection to maximize the perceived users' satisfaction as well as the MEC servers' profit based on the global view of the SDN controller [4]. Furthermore, it is expected that the flexible operation of SDN can be extended to consider end-to-end network orchestration with heterogeneous components, domains, and relevant interfaces [5]. In this way, it can be noted that SDN can provide benefits in the field of communications and computing with respect to the perspectives of both users and network operators [4]. Basic data delivery in SDN is performed by matching the flow rules in the forwarding plane nodes stored by the controller. When there is no matched rule for the incoming flow, the forwarding node asks for rule caching to the controller using the packet-in message. Then, the controller caches the flow rule to the forwarding nodes along with the appropriate path using the packet-out or flow-mod messages. This procedure is the reactive flow rule placement operation [1]. In the forwarding plane, flow rules are cached in the local Ternary Content Addressable Memory (TCAM), which is expensive and has a limited size [6]. Therefore, the efficient rule caching in the TCAM for providing flow-based fine-granular traffic control has become one of the most important research issues in SDN. Considering the rule placement problem with the capacity constraint of the forwarding node, conventional studies have provided rule compression [7], distribution [8], and energy-aware rule caching [9]. However, these studies focused on the static scenarios without considerations of the mobile environment [10]. On the other hand, SDN is attracting interest to support mobile scenarios such as software-defined Vehicular Access Networks (VANs) [11] and the Internet of Things [12]. In mobile scenarios, there can be dynamic flow rule changes due to the device's mobility features. Figure 1 shows the motivating example of the mobile scenario. Since it is a challenging task to implement virtualized wireless control functions [13], such as resource and security management (i.e., they can be implemented in the access devices (ADs) or in the controller decoupled from the ADs), considering the wireless parts of ADs will be one of our future works. Note that ADs communicate with the controller based on the SDN interface (i.e., OpenFlow [1]), receive packets from mobile nodes (MNs), and deliver them to the core network according to the flow rules. Initially, upon receiving the new packets of flow 1 and flow 2 from the MN, AD1 encapsulates the packets in a packet-in message and sends it to the controller using the OpenFlow protocol [1]. The controller then makes the rules for flow 1 and 2 to AD1. This means that two flow rules are installed by means of the reactive flow rule placement operation as described above. We assume that flow 1 is delay-sensitive and flow 2 was delay-tolerable. According to the two flow rules, the data of the two flows can be delivered through AD1. After the MN moves to AD2's area, the MN tries to start the communication with AD2 for flows 1 and 2.