# Context-Based Dynamic Meshed Backhaul Construction for 5G Heterogeneous Networks
## 3 SDN-Based Implementation As A Proof-Of-Concept
### MmWave Mesh Backhaul Resiliency
As the detection intervals are decreased, the amount of monitoring BFD packets increase and, at the same time, the packet processing latency thresholds decrease (e.g., approximately 3 ms when using a 1 ms interval). When adding data traffic in the same interface used to monitor link failures, the monitoring packets are subject to additional processing or queuing delays, which can negatively affect the delivery of monitoring packets within the allowed intervals, causing false positive link failures [39]. This phenomenon is illustrated in Figure 12, where the data rate on both interfaces of N1 is plotted over an iteration using an 1 Gbps flow, using an 1 ms and 20 ms BFD intervals. With 1 ms, whenever the primary interface of N1 is falsely set as down, often it is possible to transition to its second interface (with the cost of a short packet transmission disruption). However, it is also possible that both interfaces can be falsely set as down simultaneously, causing a total failure on the data transmission. With the 20 ms BFD interval, there are no interruptions on the ongoing traffic, as the higher interval allows a higher protection against eventual delay surges in the monitored links. In conclusion, the 10, 15 and 20 ms BFD detection intervals were selected as a configuration baseline for upcoming experiments using BFD fast failover. The second main goal of this evaluation is focused on the accessing how fast can a link recover from a failure, when using the following resilience mechanisms:  
* **Fast-Failover reconfiguration:** Initially both N1 interfaces are alive, staying active during the first 5 s, with network traffic routed through the first active group bucket i.e., N1-N2 link. The N2 interface of the N1-N2 link is then disabled by using ifconfig command after 5 s, and re-enabled after 1 s, leaving all the N1 links active until the end of the experiment. With this approach, the network traffic is sent through path 1 (N1-N2-N4), until BFD changes N1's interface state to down, and consequently sending packets through path 2 (N1-N3-N4) due to the FF configuration. The usage of path 1 usage is then resumed when the disabled interface is reactivated. A 10, 15 and 20 ms BFD monitoring intervals were used in this scenario;  
Figure 11: Measured throughput in N4 with different BFD intervals. Figure 12: Throughput over time for an 1 Gbps UDP flow. * **SDN Controller triggered reconfiguration:** Flow forwarding rules are initially installed, forwarding the traffic from N1 to N4 through N2. After 5 s, the controller modifies the forwarding rules rules in N1 and N4 to forward packets via N3. The N2 interface of the N1-N2 is then disabled during 1 s. The controller sets all the flows to the initial state after the N2 interface is reactivated. These experiments can represent a scenario where the controller has knowledge of an upcoming failure, reconfiguring the network before it happens (e.g., prediction of link disruption due to obstacle blockage). * **No Failover:** Similar initial forwarding rules are installed as previously described (traffic between N1 and N4 is forwarded through N2). However, when the N2 interface is disabled, no reconfiguration operation is triggered. For every scenario, when having a 1 Gbps flow transmitted from N1 to N4, we measured the packet transmission interruption time after the link failure. It corresponds to the interval without any received packets in N4 after the N2 interface is disabled. Table 3 presents the average interruption times. With Fast Failover and BFD, the mean values are almost 3 times the configured interval on all the scenarios. In some occasions, the internal BFD timer starts with the last echo packet sent before the link failure, resulting in values shorter than 3 times the BFD interval. Without using any mechanisms like failover, the average interruption time reaches 1.2 s approximately for every scenario, which include the link down period of 1 s. The additional delay is due to the overhead the interface needs to take to re-establish connectivity with N1 [41].