# A Comprehensive Survey of In-Band Control in SDN: Challenges and Opportunities
## 2 Related Work and Contribution of This Survey  
After reviewing the literature, there are several surveys on SDN. To begin, there are various general surveys with a high number of citations [1; 21; 22; 23; 24; 25; 26; 27; 28; 29]: all these surveys have in common that they were published at the same time as the SDN wave. They cover the challenges and new aspects that SDN brought and how it shattered the conventional networking paradigm.  
As a technology becomes trendy in a field, challenges, opportunities and threats emerge. Thus, as SDN networks emerged, so did works [30; 31; 32; 33] that addressed the security  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Feature** & **Out-of-Band Control** & **In-Band Control** \\ \hline SDN device configuration & Simple & Complex \\ Control channel security & Safe, isolated channel & Risky, shared channel \\ Network maintenance and deployment costs & High & Low \\ Scalability & Poor & Good \\ Resilience & Costly (re-wire) & Fast failure recovery (easy traffic \\ \hline \hline \end{tabular}
\end{table}
Table 1: Features of in-band and out-of-band control.  
issues of such networks. One very controversial aspect is the centralisation of all network control in a single entity. In these works, they indicate lines of work, challenges, risks, threats, and works that propose countermeasures to deal with the most common threats.  
As soon as the new technology stops being trendy and starts to be implemented in the industry, it is necessary to start studying how to make the transition from the old paradigm to the SDN paradigm. It is therefore necessary to study and analyse the necessary intermediate steps, as it is impossible to replace all legacy equipment in a single step. This is where hybrid SDN architectures and the corresponding surveys where they are studied in detail come in [11; 34; 35].  
Another aspect that has been addressed [36; 37] in the SDN field is distributed systems where more than one control entity come into play to manage a group of SDN nodes. These systems solve many of the performance problems, bottlenecks, and threats that arise from centralisation, but they also bring with them challenges in managing shared resources, such as race conditions, and atomicity in instantiating rules in SDN nodes, among others. Finally, another topic that has been deeply explored [38; 39] is the integration of other trendy technologies such as IoT with SDN, what challenges arise in the integration, and what benefits the IoT environments might derive from the SDN paradigm. Additionally, other specialised surveys analyse SDN programming languages [40], link failure recovery [41], or Quality of Service (QoS) [42].  
The aforementioned works represent only a small part of the total number of surveys on SDN. Our intention is to highlight some of the most referenced works and the topics they cover, while emphasising that an analysis of all existing surveys on SDN is beyond the scope of this article due to their large number. All the publications mentioned above have in common that they do not cover the in-band control paradigm in depth. For this reason, this review is of particular interest as it brings together all the advances in this area in a single work. Nevertheless, to date and to the best of our knowledge, there has been no survey of in-band control in SDN, which is remarkable given its importance for real SDN deployments.  
Finally, we would like to clarify that in-band control is not the same (and not even directly related) to in-band network telemetry [43]. Although the latter also emerged with the growth of SDN and programmable networks, in-band network telemetry is a technique that uses the data plane to directly drive the network measurement process, while in-band control is a model to deploy control channels leveraging the existing data plane channels. In summary, in-band telemetry is related with data plane devices (hardware), while in-band control is related with data plane channels/link (logic).