# A Comprehensive Survey of In-Band Control in SDN: Challenges and Opportunities
## 4 A Survey of In-Band SDN Control in SDN
### Preliminary Survey Overview and Classification  
Before studying each of the surveyed works, we decided first to provide an overview with patterns and similarities found in the articles. Among the motivations for each design, we discovered the main following ones, which are illustrated in Figure 5 as four categories:  
* Automatic routing;
* Fast failure recovery;
* Network bootstrapping;
* Distributed control. Figure 4: Number of papers related to SDN in-band and their amount of citations. Figure 5: Main categories of the survey. The three first categories could be seen as three main features that in-band control should provide, while the fourth is an orthogonal aspect. More specifically, the need for **automatic routing** in in-band control seems quite straightforward: while out-of-band control is usually implemented with direct links towards the controller (although it should not be necessarily like that), in-band control requires calculation of routes between SDN devices and the controller/s and vice versa. **Fast failure recovery** could be considered one of the main advantages of in-band; by simply selecting a different route in the data plane, the control communication can be recovered after a link or switch failure. However, the procedure to build these new routes should ideally be defined in accordance with the routing mechanism. Regarding **network bootstrapping**, it refers to the ability of setting up all required parameters (such as the connectivity details for the controller-switch communication) prior to startup of the system. Network bootstrapping is also a desirable feature for out-of-band control, but it is almost a must for in-band, since routes might vary depending on each deployment and the routing method applied. Finally, if the network has **distributed control** (or controllers), it implies that two or more controllers are involved in the control decisions and, therefore, a coordination mechanism should be defined for routing, failure recovery, and bootstrapping. Additionally, in-band control can also serve as a potential means for internal communication among controllers. The previous four aspects were the most commonly found. For that reason, they were selected as the four main classification items of our survey. Nevertheless, some other features were also found in the literature, such as **topology discovery**, the possibility of combining both out-of-band and in-band as a **hybrid-band** deployment, and other design parameters such as scalability or QoS. Although network topology discovery is a key element for a number of necessary network functionalities, it was decided to not include it as a key aspect to be analysed in the survey. This is because, in general, it is usually a process that is independent of the in-band protocols, as some solutions include a topology discovery system/function, while others rely on independent topology discovery protocols that are used to obtain the necessary paths to compute the in-band routes. To delve into these aspects, let us consider Figure 2 again. The network represented in the figure contains seven SDN switches and two SDN controllers. The controllers are distributed and control type is hybrid-band, as both types are combined. For example, Controller A is connected to switches 1 and 3 via out-of-band control, but also to 4 and 7 via in-band control. The case of Controller B is similar, with some switches connected via out-of-band control, other ones via in-band; there are even some combining both option, such as switches 1 and 2, which can connect to Controller B using both types of control. In this scenario, fast failure recovery can be implemented via restoration or protection (as defined by [PERSON] et al. [44]). For instance, switch 1 could be connected to Controller A and Controller B at the same time via out-of-band control, while maintaining a third in-band control route to Controller B routed through switch 2 for enhanced resiliency.