# QoS-Aware Flexible Handover Management in Software-Defined Mobile Networks
## 3 QoS-Based Flexible Handover Scheme in SDN
### QoS-Based Flexible Handover Procedures in SDN  
Based on the classification described in 3.1, QoS-based flexible handover procedures can be provided and controlled by the SDN controller actively or proactively whenever a handover event happens. The proposed scheme aims to operate the differential handover procedures according to the class of handover flows to guarantee the different QoS requirements of classes and efficiently utilize network resources. Figure 1 shows an access network architecture of an SDN-based mobile network in which the access switches (ASs) and core switch (CS) are deployed in the forwarding plane and the controller is in the control plane. In this paper, wireless parts considering wireless features are not covered. Extending the proposed scheme to the wireless parts will be the subject of one of our future works. The AS is the first switch to which the mobile node (MN) attaches, and the CS is connected with several ASs and located in the switching point of paths when MN moves between ASs. In addition, all the forwarding plane nodes are controlled by the SDN controller using a specific interface (e.g., OpenFlow, ForCES) to add, modify and delete the flow entries (FEs) of each node (for scalability, it can be noted that the SDN \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Class Type** & **Delay Tolerant** & **Loss Tolerant** & **Examples** \\ \hline Class 1 & No & No & Best effort services \\ Class 2 & No & Yes & FTP, E-mail \\ Class 3 & Yes & Yes & Interactive game, video telephony \\ Class 4 & Yes & No & VoIP \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Flow classifications. FTP: file transfer protocol; VoIP: Voice over IP. controller in the proposed scheme is only in charge of the access networks [10]). When the entry is added for a flow, we assume that both uplink (UL) and downlink (DL) flow entries are set at the same time if not otherwise noted. By using this interface, each forwarding node sends statistics reports which include the statistical information of each flow or aggregated flows. The proposed scheme utilizes these statistics reports to help controller recognize the L2 handover trigger of MN. For example, ASs can recognize the L2 handover trigger when the specific flow entry counter that shows how many times the flow entry has been utilized decreases under the specific threshold. Then, the ASs can transmit the statistics report to the controller. As a result of this statistics report, the controller can observe the L2 handover trigger of MN. On the other hand, from the statistics report when the specific flow entry counter exceeds a specific value after the handover, the controller can recognize the MN's attachment to the new AS (if the access points (APs) also support the SDN interface, the L2 handover trigger can be easily found from the APs, and this information can be informed to the SDN controller [48]). Additionally, in order to make MN unaware of the mobility, the router solicitation (RS) and router advertisement (RA) messages are exchanged between the MN and the controller in the SDN-based architecture. Based on the explanations above, there are four types of handover procedure for the QoS-based flexible handover scheme based on the flow classification [14]: (1) the reactive procedure for Class 1 flows, (2) the buffering support procedure for Class 2 flows, (3) the buffering support and proactive procedure for Class 3 flows, and (4) the proactive procedure for Class 4 flows. Figure 2 shows the information flows for the proposed scheme. Although there can be several candidate ASs for handover, only AS 2 is described in the figure for simplicity. The detailed operation of each handover procedure is described as follows. Before the MN moves, the MN's data is delivered via AS 1 and CS through matching to the flow entry at each node. In the flow table of AS 1, the flow entries of the flows in Classes 2, 3 and 4 are set to transmit the statistics reports to the controller for the L2 handover trigger, as mentioned above.