# QoS-Aware Flexible Handover Management in Software-Defined Mobile Networks
## 1 Introduction  
Along with the emergence of various quality-of-service (QoS)-sensitive applications and wireless mobile devices, mobility support has been an important issue for over two decades in the network area. Especially in today's all-IP mobile networks which combine the Internet and telecommunication networks, IP-based mobility management has been applied to enable mobile users to continue their communications even when they move freely. Since IP was originally designed for communications between fixed and static devices, a large number of works on IP-based mobility protocols have been conducted to develop mobility support for IP, such as mobile IP (MIP), hierarchical MIP (HMIP), fast handover for MIP (FMIP) and proxy MIP (PMIP) [1]. However, these conventional protocols only concentrate on maintaining IP session connectivity without considering whether QoS is guaranteed during handover. For example, handover delay and packet loss during binding updates and path changes can cause a QoS degradation in QoS-sensitive service flows [2; 3]. Although related researches such as proactive handover and buffering schemes have been utilized to cover these problems [2; 3; 4], they cannot also consider various QoS requirements of services, which means that each solution is limited only to a specific service. In addition, there have been works undertaken to extend the Internet QoS paradigms, such as integrated services (IntServ) [5] and differentiated services (DiffServ) [6], to the mobile IP protocols [7; 8]; however, they did not consider the handover performance, such as handover delay and packet loss, because they arose from the path reservation. Cellular telecommunications networks, meanwhile, should guarantee the QoS requirements of users when they move at any time because the fundamental principal of the cellular networks is to provide mobile services to users. Therefore, in the current 4G networks which provide all-IP services based on the IP mobility protocols, additional mechanisms such as (direct or indirect) tunneling between base stations (BSs), buffering and proactive resource reservations are utilized to satisfy the principal requirements of mobility support [9]. For example, 4G networks provide almost seamless and loss-less handover through the tunneling between eNodeBs (eNBs) (direct tunnel between eNBs or indirect tunnel via a serving gateway (SGW)), buffering and proactive resource allocation controlled by the mobility management entity (MME) [4]. However, an overly strict handover mechanism for all kinds of service flows in the cellular networks can result in inefficiencies such as signaling overheads and unnecessary resource reservations from the network operators' view [10]. As mentioned above, even though the services have become more diverse and have their own specific QoS requirements, conventional works have only provided a unitary handover mechanism for all kinds of service flows, irrespective of the different QoS requirements. From the network operators' view, flexible handover mechanisms according to the service properties are required for efficient network resource utilization as well as QoS support. However, it is difficult to adopt QoS-aware flexibility in today's IP-based network because the network usually operates simply according to IP address routing. Moreover, since the network architecture depends on the vendor/hardware and operates dispersively, the network operators cannot flexibly configure the network nodes and implement their own network policy [11]. On these grounds, software-defined networking (SDN) has emerged, which separates the control plane from the forwarding plane with a programmable interface between them (e.g., OpenFlow [12], ForCES [13]). The control/forwarding separation in SDN allows for logically centralized intelligence with a global view in the control plane and flow-based simple packet forwarding in the forwarding plane.