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==SDN control plane==
The implementation of the SDN control plane can follow a centralized, hierarchical, or decentralized design. Initial SDN control plane proposals focused on a centralized solution, where a single control entity had a global view of the network. While this simplifies the implementation of the control logic, it has scalability limitations as the size and dynamics of the network increase. To overcome these limitations, hierarchical and distributed approaches have been proposed. In hierarchical solutions,<ref name= Yeganeh>{{cite journal|first1=S.H.|last1=Yeganeh|first2=Y.|last2=Ganjali|title=Kandoo: A Framework for Efficient and Scalable Offloading of Control Applications|doi=10.1145/2342441.2342446|s2cid=193153|doi-access=free}}</ref><ref name= Ahmed>{{cite journal|first1=R.|last1=Ahmed|first2=R.|last2=Boutaba|title=Design considerations for managing wide area software defined networks|journal=IEEE Communications Magazine|year=2014|volume=52|issue=7|pages=116–123|doi=10.1109/MCOM.2014.6852092|s2cid=7912785}}</ref> controllers operate on a partitioned network view, while decisions that require network-wide knowledge are taken by a logically centralized root controller. In distributed approaches,<ref name= Koponen>{{cite journal|url=https://www.usenix.org/legacy/events/osdi10/tech/full_papers/Koponen.pdf|first1=T.|last1=Koponen|title=Onix: A Distributed Control Platform for Large scale Production Networks|journal=Proceedings USENIX, Ser. OSDI'10|___location=Vancouver, Canada|year=2010}}</ref><ref name= Tuncer1>{{cite journal|title=Adaptive Resource Management and Control in Software Defined Networks|journal=IEEE Transactions on Network and Service Management|volume=12|issue=1|pages=18–33|date=March 2015|doi=10.1109/TNSM.2015.2402752|last1=Tuncer|first1=Daphne|last2=Charalambides|first2=Marinos|last3=Clayman|first3=Stuart|last4=Pavlou|first4=George|hdl=10044/1/63600|s2cid=9215618|url=https://discovery.ucl.ac.uk/id/eprint/1478892/|hdl-access=free}}</ref> controllers operate on their local view or they may exchange synchronization messages to enhance their knowledge. Distributed solutions are more suitable for supporting adaptive SDN applications.{{clarify|reason=what is an adaptive SDN application?|date=June 2025}}
A key issue when designing a distributed SDN control plane is to decide on the number and placement of control entities. An important parameter to consider while doing so is the [[network delay|propagation delay]] between the controllers and the network devices,<ref name= Heller>{{cite book|first1=B.|last1=Heller|first2=R.|last2=Sherwood|first3=N.|last3=McKeown|title=Proceedings of the first workshop on Hot topics in software defined networks - HotSDN '12|chapter=The Controller Placement Problem|year=2012|page=7|doi=10.1145/2342441.2342444|isbn=9781450314770|s2cid=1770114 }}</ref> especially in the context of large networks. Other objectives that have been considered involve control path [[Reliability (computer networking)|reliability]],<ref name= Hu>{{cite journal|title=On the placement of controllers in software-defined networks|year=2012|doi=10.1016/S1005-8885(11)60438-X|last1=Hu|first1=Yan-nan|last2=Wang|first2=Wen-Dong|last3=Gong|first3=Xiang-Yang|last4=Que|first4=Xi-Rong|last5=Cheng|first5=Shi-Duan|journal=The Journal of China Universities of Posts and Telecommunications|volume=19|pages=92–171}}</ref> [[fault tolerance]],<ref name= Ros>{{cite book|chapter=Five nines of southbound reliability in software-defined networks|year=2014|doi=10.1145/2620728.2620752|last1=Ros|first1=Francisco Javier|last2=Ruiz|first2=Pedro Miguel|title=Proceedings of the third workshop on Hot topics in software defined networking |pages=31–36|isbn=9781450329897|s2cid=17088018}}</ref> and application requirements.<ref name= Tuncer2>{{cite book|chapter=On the Placement of Management and Control Functionality in Software Defined Networks|doi=10.1109/CNSM.2015.7367383|title=2015 11th International Conference on Network and Service Management (CNSM)|year=2015|last1=Tuncer|first1=Daphne|last2=Charalambides|first2=Marinos|last3=Clayman|first3=Stuart|last4=Pavlou|first4=George|pages=360–365|isbn=978-3-9018-8277-7|s2cid=6977724|url=https://discovery.ucl.ac.uk/id/eprint/1502968/}}</ref><!--[[User:Kvng/RTH]]-->
==SDN Data Plane==
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