Hybrid SDN Performance: Switching between Centralized and Distributed Modes under Unreliable Control Communication Channels +
Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/cne](https://creativecommons.org/licenses/by/cne)]([https://creativecommons.org/licenses/by/cne](https://creativecommons.org/licenses/by/cne))  
are being designed by various groups/organizations (e.g., ETSI Network Function Virtualization), which aim at an end-to-end orchestration of all network segments from RAN to core all the way through transport. On the transport side (the focus of this paper), Software-Defined Networking (SDN) has also been integrated in these frameworks [4] as a fundamental network programmability enabler. The SDN paradigm separates the data and control planes of the network. It logically centralizes the control of a network in an SDN controller which acts as a brain of the network and is in charge of telling each network node how to forward incoming packets by installing the appropriate forwarding rules. One of the main advantages it brings is programmability through this single entity (the logical controller) with which network management applications must interact to apply their policies. Through agreed-upon APIs, the full potential of SDN can be exploited by the network managers. This has been the case in controlled scenarios that do not pose challenging constraints to the deployment of such a paradigm because of the availability of reliable and high capacity networks, for instance, in data centers. However, moving to wide area networks (WAN) (e.g., microwave or mm-Wave backhaul links) where in-band channels are responsible to carry control traffic between the nodes and the SDN controller, the assumption of the availability of such a reliable network may not hold anymore as performance of the wireless link changes with the environmental conditions, which leads to a high risk of experiencing channel impairments, which might cause centralized SDN operation failure by affecting communication between the transport component of SCs and the SDN controller. Therefore, although SDN provides huge advantages for managing networks by splitting the control logic, there are some pitfalls of such separation in the presence of unreliable conditions, namely (i) _data plane faults_ (the network element(s) or port(s) associated to the network element(s) fails), (ii) _control channel faults_ (the connection between the SDN controller and the data plane element(s) fails or it experiences losses), and (iii) _SDN controller faults_ (the SDN controller fails). For instance, when using OpenFlow [5], control messages that are fundamental for installing packet forwarding rules (e.g., PacketIn and PacketOut) and node handling (e.g., PortStatus) or topology discovery messages (Link Layer Discovery Protocol, or LLDP) may be lost. TCP retransmissions may be able to handle some low loss situations; however, a more detailed analysis is needed. Therefore, handling such impairments is key for the correct operation of the transport network in such environments. Previous work explored the application of SDN to wireless transport networks [6, 7, 8], though it is generally assumed that unreliability and failures may only happen in the data plane. In fact, previous studies mainly focused on data plane reliability by designing efficient schemes for fast detection and recovery of the communication [9, 10, 11]. Controller-only schemes have also been integrated, for instance in ONOS, in which complex cluster management and device mastership procedures were put in place toward consistency in the case of the controller failure [12]. Additionally, hSDN approaches have also appeared for allowing the transition of networks from distributed to SDN-based operation [13], which also offers interesting ideas that can be applied to our problem.