# A Comprehensive Survey of In-Band Control in SDN: Challenges and Opportunities
## 1 Introduction  
During the last decade, the flourishing of SDN [1] has led to a wide range of novel network services and applications, including inter-disciplinary use cases, such as those envisioned by the fifth generation of mobile technologies (5G) and beyond [2]. The cornerstone of SDN is the possibility to separate the control and data planes, hence providing multiple advantages in network management and operational costs, among others. Uncoupling both planes involves the utilization of a control communication protocol that is in charge of synchronising both planes. Considering the SDN architecture, this protocol is part of the Southbound Interface (SBI), formally known as Data-Controller Plane Interface (D-CPI), as defined by the Open Networking Foundation (ONF) [3]. Although the control protocol that initiated the SDN paradigm is OpenFlow [4], others currently exist, such as P4 Runtime [5]; legacy ones (e.g., Simple Network Management Protocol (SNMP)) are even considered potential control protocols in the generalised SDN architectural model. When deploying the control channel, it is possible to follow either an out-of-band or in-band approach, as depicted in Figure 1. On the one hand, the channel is out-of-band when the physical links leveraged for communication are different for the control and data planes, as illustrated in Figure 1a. In this scenario, the network requires at least one extraphysical link per SDN device, apart from the data plane topology, for the control channel. On the other hand, the channel is in-band when physical links are shared between the control and data planes and no additional links are required. Additionally, hybrid-band control is defined when some links are shared but, at the same time, dedicated links for control also exist [6], as in the example shown in Figure 2.  
Each type of deployments has its own advantages [6], and the selected approach mainly depends on the network scenario [7; 8; 9]. Some of the disadvantages of out-of-band are advantages of in-band and vice versa, while hybrid-band could be considered a trade-off among both approaches. We summarised the main features of each model in Table 1:  
Figure 1: SDN out-of-band and in-band control networks. Figure 2: SDN hybrid-band control network. The current trend shows that in-band seems to be prominently used in SDN deployments [10], particularly for large networks in which the cost of using out-of-band might be exorbitant. Additionally, in-band control allows a wide range of new applications, particularly for constrained and hybrid SDN environments [11; 12], in which out-of-band control is complex (or even impossible) to deploy. Some of the use cases leveraging in-band control include 5G [13] and Non-Terrestrial Networks (NTNs) [14], Internet of Things (IoT) (e.g., underwater networks [15], energy saving [16] or resource-constrained environments [17]), emergency and disaster situations [18; 19], and enhanced network security [20]. Nevertheless, despite of the multiple benefits of in-band control in SDN scenarios, few efforts have been made to design a common and holistic in-band control protocol. This design should consider standard and popular platforms (both for SDN controllers and switches/devices) for a complete integration in current SDN deployments. Therefore, the aim of this survey is to provide the current state of the art in this area, describing current proposals, technologies used, advantages and disadvantages, etc., so that it can serve as a comprehensive report for researchers and network managers interested in this topic. Therefore, our goal is to provide a comprehensive survey of the topic and a comprehensive overview for academia and industry. Accordingly, the main contributions of our survey are  
* A comprehensive list of works about in-band SDN control proposals. * A classification of the surveyed works in terms of designed features, defining four categories. * A chronological table per category stating achieved functionality and maturity of each proposal.