# Transport-Layer Limitations for NFV Orchestration in Resource-Constrained Aerial Networks
## 5 Conclusions  
Management and orchestration of infrastructure resources and virtual functions are fundamental to coordinate the operation of NFV environments. The proliferation of new enabling technologies in the telecommunications market, such as SUAVs, creates new opportunities for the fast and cost-effective deployment of network services following the novel NFV paradigm. However, this opportunity opens new challenges and hurdles to NFV orchestration: (1) the limited lifetime of NVFI nodes, which has to be taken into account for migrating nodes and functions in replacement cases; (2) the intermittent availability of control communications, which can make nodes unavailable for communications in short periods of time; (3) the limited capacity of NFVI nodes, which affects the load an SUAV is able to carry and the protocol used for exchanging information; (4) the transport-layer protocols for control communications, whose performance may be decreased in mobile ad-hoc networks, as in the case of TCP; and (5) the support of enhanced policies for VNF placement, which current VIMs do not provide as they are not aware of SUAV specific aspects, such as battery constraints and geographic location [6].  
In this paper, we performed a set of simulations to check the behavior of orchestration traffic when TCP is used as the underlaying transport-level protocol. To do so, we defined some tests with a set of SUAVs forming a grid topology where aircraft had to be replaced after some period of time, simulating some battery depletion and subsequent replacement. Afterwards, we took a look at the average throughput to check how intermittencies affected the orchestration traffic performance. Our results show that the number of hops a SUAV is from the GCS has a direct impact in the throughput of the orchestration traffic exchanged with the SUAV, since the longer the path is the more likely that routing paths will be broken. However, this is not the only reason that may cause performance degradation. The number of neighbors of a SUAV in the aerial network (that is, the SUAV degree) also correlates with the performance of orchestration traffic, since having less nodes around an aircraft could entirely sever a TCP connection if the routing protocol does not converge before the backoff timers from TCP expire. This is a problematic issue that needs to be considered by MANO platform implementations operating on network environments such as those considered in this paper. Moreover, under close to congestion conditions this effect is much more pronounced, and UDP is able to provide a more stable throughput as compared to TCP, enabling more time periods where the SUAV nodes and the GCS receive the control, while TCP has more idle periods that hurdle NFV orchestration actions over the aerial network.  
Our future work includes the implementation of our SUAV reference scenario. For this purpose, we will use an emulation platform for multi-UAV applications that we are developing [46]. In addition, we will continue developing solutions to address the rest of the challenges to NFV orchestration over resource-constrained environments that we have identified in this paper.  
**Author Contributions:** Funding acquisition, F.V.; Investigation, L.F.G., I.V. and F.V.; Supervision, I.V., F.V. and D.R.L.; Validation, L.F.G., B.N. and V.S.-A.; Writing--original draft, L.F.G. and I.V.; Writing--review & editing, L.F.G., I.V., F.V., B.N., V.S.-A. and D.R.L.  
**Funding:** This article has been partially supported by the European H2020 5 GRANGE project (grant agreement 77/137), and by the 5 GCity project (TEC2016-76795-C6-3-R) funded by the Spanish Ministry of Economy and Competitiveness.  
**Conflicts of Interest:** The authors declare no conflict of interest.