# A Comprehensive Survey of In-Band Control in SDN: Challenges and Opportunities
## 4 A Survey of In-Band SDN Control in SDN
### Automatic Routing
They achieve greater efficiency in the use of the controller's resources than with a static approach. [PERSON] et al. [67] present Izzy, a distributed routing protocol for maintaining robust connectivity of in-band SDN control channels. Izzy is based on a combination of a spanning tree and topology-dependent temporary addresses called _locators_. The solution is divided into three modules; the first one, called TREE, is responsible for maintaining the spanning tree structure by assigning locators. The second module, known as UPDATER, is \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Article** & **Year** & **Key Features** & **Evaluation \& Tools** & **Available** & **Overall** \\ \hline
**[PERSON]** & 2016 & Key ideas for in-band control with DHCP, queueing and failure recovery & Emulation: NOX/BOFUSS/Pan-European, star networks & — & \\
**[PERSON]** & 2017 & Routing and single-link failure recovery & Analytical model & — & \\
**[PERSON]** & 2017 & Routing and link failure recovery & Analytical model & — & \\
**[PERSON] et al. [63]** & 2017 & Routing and link failure recovery & Analytical model/Random networks & — & \\
**[PERSON]** & 2018 & Routing and link failure recovery & Emulation: Ryu/OVS/Mesh network & — & \\
**[PERSON]** & 2018 & Routing and link failure recovery with a & Analytical model/Internet topologies & — & \\
**[PERSON]** & 2018 & Routing and link failure recovery & Emulation: POX/OVS/Ring-tree topology & — & \\
**[PERSON]** & 2019 & Distributed routing protocol for in-band SDN control & Simulation: OMNeT++/WAN/Zoo topology & — & \\
**[PERSON] et al. [68]** & 2019 & Path selection algorithm for routing using MPTCP & Simulation: C++/BRITE networks & — & \\
**[PERSON] et al. [69]** & 2020 & Routing, in-band connection with QoS networks & Simulation: Python Networkx. Emulation: ONOS/OVS/Docker/Fat tree and random networks & — & \\
**Ningyuan** & 2021 & Routing and link failure recovery in LE networks & Simulation: Walker–Delta constellations & — & \\
**Kumazoe** & 2022 & Routing in P4 BMv2-based network & Real testbed: BMv2/Linear network & — & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary table of in-band proposals that support automatic routing. responsible for coordinating and updating the network's forwarding tables. Finally, the Fast ReRoute (FRR) module is in charge of maintaining backup routes. The evaluation of Izzy has been carried out in OMNeT++ and shows recovery times under 100 ms in WAN topologies of 100 nodes. **[PERSON]** et al. [68] define an algorithm to create robust control paths when leveraging MPTCP for in-band communication in SDN. Their algorithm looks for the best set of switches to route through the shortest and disjoint paths in the network. They compute the sum of disjoint path lengths and computation time and compare their proposal with two baseline algorithms, proving better results. However, they only test it via simulation and leave the real implementation as future work. **[PERSON]** et al. [69] propose a centralised routing solution to improve the QoS of in-band control traffic in terms of bandwidth. They analyse the in-band control traffic, finding out that it is periodic and needs a very small bandwidth: around 120 kbits every 5 s. Hence, they place the in-band connection on the path with the most available bandwidth. They design a centralised routing algorithm in the SDN controller to compute the optimal path from the network topology and current consumed bandwidth at every link. However, their simulations show that it does not scale to generic large networks. They choose to stick to a Fat Tree Data Center (DC) network and a special network designed with higher connectivity and randomness, in which they clearly improve the Round-Trip Time (RTT) and the Packet Loss Ratio (PLR) compared with a L2 Spanning Tree Protocol (STP). **[PERSON]** et al.