# Transport-Layer Limitations for NFV Orchestration in Resource-Constrained Aerial Networks
## 4 Practical Evaluation of Transport-Layer Options
### Orchestration Traffic Performance Using TCP
However, if we choose another aircraft with a lower degree by two nodes, for example SUAV (1,4), then forwarding traffic becomes a far more difficult task for the unit since the possible combinations are fewer, making it easier to find both aircraft offline, preventing the unit from receiving/sending traffic until one of those nodes is back online and the routes converge again. It is easy to spot in this example that, even though they have the same number of hops to a directly connected node to the GCS, SUAV (1,4) has almost 10% less throughput compared to (3,3), which is a noticeable difference in quality. In the worst case, if those nodes cannot be replaced on time before all TCP timers expire, then the aircraft will close the connection until it is replaced again and, in consequence, will close any orchestration communicationsbetween the SUAV and GCS until its eventual replacement, severely harm its orchestration efficiency. This is the reason the borders of the network often have worse throughput values compared to those located at the center, despite having the same number of hops to the GCS. Moreover, it is especially noticeable in the topology corners, since those are the ones with the least number of surrounding aircraft, with the worst values of the whole scenario in consequence. This problem could be amplified in other topology types, e.g., a serial topology, since they are connected along a single path: if any aircraft turns off when its battery depletes, then the risk of closing all TCP connection increases if the SUAV is not able to be changed on time, preventing the node from receiving control communications during a long time span and severely harming its performance. Figure 3: **(a) GCS-to-UAV orchestration traffic throughput in kbps. (b) UAV-to-GCS orchestration traffic throughput in kbps. It is easier to spot these reasons when looking at how orchestration traffic evolves in one of the simulation runs. Figure 4 collects the received throughput in the GCS from SUAV (4,4) in one of those simulations, since it is the one that will bring the least favorable results, i.e., where improvements are necessary to increase its performance. As it can be spotted in Figure 4, there are time periods where its throughput drops throughout the simulation, as OLSR routing needs to converge after one (or several) links are broken due to certain units being replaced. We can see that there are spikes of traffic (traffic bursts suddenly arriving at the node) because TCP sends retransmissions once there is a route recovery after a link has been broken, using the fast-recovery mechanism to send all that traffic directly once it has the opportunity, including previous retransmission received during the breakdown period. This in turn produces a less constant flow of information, and some nodes may receive repeated and outdated information at once when they arrive to the model, since its origin are retransmissions from previous queries. However, this is not the most prominent characteristic in this graph: with TCP, there are some portions of time where no orchestration traffic is present at all (around the 4000 s mark and just before reaching the 6000 s one as well). We have previously explained the reasons for this behavior: the unit cannot find a route on time before the TCP backoff algorithm expires, dropping the connection entirely, leaving the nodes without NFV orchestration during that phase until it is replaced at a later moment in time. This is the main source of throughput drops, since there are long periods without any orchestration being performed in the aircraft. This could also introduce other problems with the NFV Orchestrator, since it can interpret this connection unavailability as an issue with the nodes themselves, instead of a connection problem, leading to orchestration inefficiencies because it may think these resources are permanently lost, when in reality is just a temporary situation that will be solved after the UAV replacements and route convergence.