It is essential to support real-time communication in networked control systems, such as automotive systems or smart factories. Several studies have been conducted to schedule real-time flows by adjusting their priorities using software-defined networking (SDN). However, because its architecture was not initially designed for real-time communication, unexpected challenges may occur if it is not utilized carefully. In particular, because SDN does not provide an atomic method for priority adjustment, some flows may violate their timing constraints by receiving additional interference during priority adjustment. In this letter, we propose novel schemes to update the flow priorities safely in SDN-based real-time systems. To this end, we first analyze such problems through two types of cases. Then, we develop schemes that determine a safe procedure for priority adjustment and synchronize the priority settings of all switches. By network emulations, we demonstrate that the above problem occurs in 149 out of 300 flow sets, but our schemes can effectively prevent all of them.
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