# Transport-Layer Limitations for NFV Orchestration in Resource-Constrained Aerial Networks
## 4 Practical Evaluation of Transport-Layer Options
### Orchestration Traffic Performance Using TCP
This last problem could be fairly common in such scenarios, since some of the UAVs can be substituted simultaneously and have relatively long replacement times in certain situations, potentially leaving some aircraft without orchestration information for a long period of time, which could be avoided if another transport-level protocol is used instead. To check all the aforementioned issues, we simulated four hours of constant control communications between all the nodes and the GCS, using the battery replacement model described at the beginning of this section. To ensure consistency in our results, we run the simulation 30 times using random values for battery lifetime and replacement times at every run, obtaining the average traffic throughput reception on each node and the GCS depending on the direction of the traffic. The obtained results can be seen in Figure 3.  
As it can be observed in the heatmaps, the throughput of control communications is affected in all SUAVs with a various degree of impact depending on the position of the aircraft: when a SUAV is close to the GCS, its throughput is almost the same as if no replacements were performed, nearby the aforementioned 7 and 5 kbps mark, as in SUAVs (1,1) and (1,2), both directly communicating with the GCS. However, the further away we get from the SUAVs directly connected to the station, then the lower the throughput becomes in most cases. This jump in quality is attributed to link breakdowns: when a SUAV is directly linked with the GCS, then its connections do not depend on more aircraft to forward its traffic, which in turn will translate into an almost perfect connection. On the other hand, the longer the distance (in hops) from/to those aircraft is, the more likely it is that routing paths might break, requiring some re-routing and starting the backoff algorithm at the sender until an available path is found again. Hence, we can see that having more hops between a SUAV and the GCS will negatively impact its throughput using TCP because the number of link increases along its traffic path, increasing the chances of a path breakdown and leaving the nodes with more periods where no control communications can be performed between the GCS and the aircraft, only trying to send retransmissions following the instructions of the congestion control algorithm. The best example of this property is the difference between SUAVs (1,2) and (4,4): the throughput decrease is more than 20% in both ways, which is a very noticeable difference against its expected value. Even though this factor is true in most cases of the topology, it is also possible to spot similar throughput results on SUAVs located at the center of the heatmaps: despite being further away from the aircraft connecting with the GCS, they have very close values with their nearby neighbors, in some cases even slightly improving its performance as in the case of SUAVs (3,2) and (3,3). Moreover, nodes that should have a similar performance with the same number of hops have a wider disparity of values than expected: such is the case of (3,3) and (1,4), where their distance is 2 hops from the nearest link to the GCS (node (1,2)), but their throughput is noticeably worse in the latter case (10% decrease with respect to one another). In all these instances, it can be seen another important property on their connection health: the degree of the nodes affects its throughput as well. Routes in the network will be recomputed each time a SUAV has to be recharged; having more neighbors to forward the TCP traffic aids finding more available routes to reach the destination, while having fewer units will introduce more \"dependency\" on those surrounding units. The best way to describe this behavior is using an example: SUAV (3,3) has four nodes surrounding it, i.e., a four degree node, so it is more likely to find a route faster (since more paths are available when using its three remaining nodes), and finding all four units simultaneously down due to simultaneous replacements is very unlikely.