# Transport-Layer Limitations for NFV Orchestration in Resource-Constrained Aerial Networks
## 3 Orchestration in Intermittently Available Platforms
### Transport-Layer Protocols for Control Communications  
Continuing with the VIM-SUAVs communication, we must focus on certain aspects related with the behavior of HTTP that harms the overall performance of NFV orchestration in SUAV scenarios. HTTP uses TCP as its default transport-layer protocol. Although this protocol is one of the main pillars of telecommunications in current networks, certain works have shown that TCP performance over (mobile) ad-hoc networks makes it not the most appropriate choice for them, as it can be severely lowered compared to completely wired networks [28; 29; 30; 31; 32]. On its conception, TCP was developed as a protocol for reliable data transfer over wired networks. In most cases, link-related error would rarely occur, and most packet losses would be related to congestion. However, wireless technologies completely changed how communications could be done using more than cables to interconnect devices, allowing free node movement as well as flexibility to the distribution of a network, but also introduced new challenges that increased link-related failures due to several factors such as the instability of the medium and mobility. In consequence, TCP was never intended to deal with temporary and/or common link failures, which could potentially affect its use in wireless environments, creating new drawbacks that had to be dealt with in order to ensure efficient communication through this medium. Most of them can be found in [28; 29], but the most relevant for NFV orchestration in SUAV networks are the following ones:  
* Physical layer: Wireless links induce more errors in the packets. Problems such as fade-away and interferences can modify bits during the transmission, while poor reception/channels will create packet losses, severely impacting its performance [28]. The reason for this decrement is a constant retransmission of packets due to these losses/modifications, keeping its congestion window low and reducing the overall throughput in consequence [29]. * Network layer: Routing in ad-hoc networks has some differences over traditional networks, using protocols such as Dynamic Source Routing (DSR) [33], Ad-hoc On-Demand Distance Vector (AODV) [34] or Optimized Link-State Routing Protocol [35]. These protocols assume that nodes form dynamic topologies that may change over time. Therefore, they need some time to recompute their routes when one or more nodes become unavailable before reaching a stable state, which can be dependent on parameters such as the chosen mobility model [30]. However, the disparity between the time to recompute a route and TCP retransmission timers may cause the sender to use outdated routes, increasing the number of retransmissions performed and, again, reducing the congestion window along with its throughput. This problem is further exacerbated when the number of intermediate nodes is increased, as they must also recompute routes as well [31]. Most of the problems described above can be summarized in one sentence: TCP is not able to recognize the source of the failure in wireless media. Therefore, it invokes the only mechanism to, in theory, increase overall transmission performance: the congestion control mechanism. However, here it is disadvantageous because, in almost all cases, there is no congestion in the network, but the protocol interprets SUAV disconnections as a congestion problem even though it is not the case. Hence, this reaction will decrease throughput, harming overall performance instead of increasing it since there is no congestion in the network, as the control information is usually not high in comparison with other types of data exchange between nodes. Although TCP might not be the most suitable option to transmit information in SUAV environments, we still want some of its characteristics when transmitting control information in a NFV context, as it is vital to ensure reliable data transfer to every node inside the network.