# QoS-Aware Flexible Handover Management in Software-Defined Mobile Networks
## 2 Related Works
In the SDN-based DMM approach, mobility is achieved by the configuration of the forwarding rule on the access routers (DMM gateways) controlled by the SDN controller to redirect the traffic to the new access routers and make the users' IP address unchanged from the user's perspective after handover. However, these works also only consider static mobility management without considering the QoS requirements on the flows. In the SDN-based DMM approach, handover and new sessions are only differentiated in the path assignment, and various QoS properties have not been considered. Although there have been research works undertaken on QoS provisioning in SDN, these works have focused on flow admission control to guarantee the QoS requirements utilizing the given resources of the forwarding plane nodes [25; 26; 27] and controllers [28]. In other words, previous works have been limited to admission control and resource allocation to the incoming flows, indicating that the QoS of the flows and QoS-based mobility management during network operations have not yet been introduced in SDN research. On the other hand, in order to provide reliable network services in SDN, reliability and availability issues have been practically researched [29; 30]. Based on the SDN architecture, these issues can be classified into data plane, control plane and application issues. In the data plane, abnormal behaviors in packet delivery have been covered [31; 32], such as reachability failure (broken pipe), forwarding loops and tunneling errors. In the case of the control plane, fault diagnosis and controller state inconsistency have been studied [33; 34]. Application plane research works have included incorrect implementation issues [35]. In addition, there have been research works on the monitoring system in SDN, which is a fundamental requirement for the network management system [36]. The monitoring system is necessary to provide accurate and timely resource reconfiguration [37], intrusion detection [38] and end-to-end path measurement [39]. Although the current paper does not cover these resilience issues and network monitoring systems, considering the data/control plane resilience issues based on the monitoring system during handover will be the topic of one of our future works. In our previous work [14], a QoS-based mobility management scheme was initially introduced. However, the previous work had some limitations that are highlighted as follows. Firstly, the previous work assumed that all the forwarding nodes support the mobility management protocol to play the anchor role and perform the binding updates because each node can be a candidate crossover node between two paths during handover. This is because each forwarding node has a tightly coupled architecture between the control and forwarding plane. As a result, all nodes should be upgraded or modified to provide the mobility management protocol, which can be a critical limitation in terms of cost. Secondly, the previous work also assumed that the crossover forwarding nodes could intercept the handover initiation message based on the mobile users' predictive next location in the layer 2 (L2) trigger message. However, it is difficult to predict the mobile user's next location practically through the layer 2 (L2) trigger in Wi-Fi networks without modifications on the users' device. These limitations can be solved in the SDN-based networks. In SDN, the network intelligence to provide mobility management exists only in the control plane, which is decoupled from the forwarding plane, where each node only performs packet forwarding. In addition, the candidate next locations of mobile users can be determined based on the global network view of the control plane. In this paper, in order to overcome the limits of the previous general handover management system [11; 19; 22], as mentioned above, a QoS-based flexible handover management scheme in SDN is proposed which provides differential handover procedures based on the QoS requirements.