# A Comprehensive Survey of In-Band Control in SDN: Challenges and Opportunities
## 6 Conclusions  
In this survey, we examined all existing works in relation to in-band control in SDN. We also showed that there was some tendency in the literature for the referenced keywords SDN and in-band to grow from 2012 to 2018 and stay stable in the later years.  
According to the analysis, we classified them in four main categories: (1) automatic routing: in-band control requires calculation of routes between SDN devices and the controller/s and vice versa, (2) fast failure recovery: by selecting a different route in the data plane, the control communication can be recovered after a link or switch failure, (3) network bootstrapping: refers to the ability of setting up all required parameters before starting the system, and (4) distributed control: which 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.  
In order to provide a comprehensive overview, we have summarised them in one table per category, including a qualitative score based on their overall contribution. These tables show the year of publication of the article, its main characteristics, the evaluation tools, whether its code is publicly available, and an overall contribution based on three stars, which was qualitatively assigned on the basis of all the previous characteristics, together with its impact in terms of number of citations.  
According to our investigation, research efforts are still needed for scalable fast failure recovery and the use of in-band in distributed control scenarios. Another line of research to be promoted is the application of ML to improve in-band routing links, network security, and failure recovery.  
As future research trends and challenges, we envision the importance of IoT in which some components might be constrained in energy or battery and out-of-band control is unfeasible, leveraging in-band as an EWBI protocol for controller clusters, and research on tackling potential security risks. Furthermore, most proposals cover partial aspects or are not fully integrated into real SDN platforms and devices. Therefore, there is still a need for a holistic in-band control design integrated into an industry-driven SDN framework.  
Conceptualization, E.R. and J.M.A.; methodology, J.M.A. and E.R.; investigation, E.R., J.M.A. and D.C.; resources, J.M.A.; papers searching and selecting, E.R. and J.M.A.; writing-original draft preparation, E.R., J.M.A. and D.C.; writing--review and editing, J.A.C., D.L.-P. and E.R.; visualization, D.L.-P. and J.A.-H.; supervision, E.R. and J.M.A.; project administration, E.R. and J.M.A.; funding acquisition, E.R. and J.A.C. All authors have read and agreed to the published version of the manuscript.  
This work was funded by grants from Comunidad de Madrid through projects TAPIR-CM (S2018/TCS-4496) and MistLETOE-CM (CM/JIN/2021-006), and by project ONENESS (PID2020-116361 RA-I00) of the Spanish Ministry of Science and Innovation.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
5G The fifth generation of mobile technologies  
6 TiSCH IPv6 over the TSCH mode of IEEE 802.15.4e ARP Address Resolution Protocol  
BOFUSS Basic OpenFlow Userspace Software Switch  
BMv2 Behavioral Model version 2  
DC Data Center  
DHCP Dynamic Host Configuration Protocol  
D-CPI Data-Controller Plane Interface  
EWBI East/Westbound Interface