As an innovative network architecture decoupling data forwarding and control logic

and opening underlying programming interfaces

software-defined networking (SDN) provides a novel solution to reduce deployment and operation costs and improve business application performance in core networks. However

logically centralized control plane under the SDN architecture is prone to performance bottlenecks

and increases packet forwarding delay. Thus it is necessary t

o understand the characteristics of its packet forwarding performance. To this end

we first introduce typical deployment scenarios of software-defined core networks

and analyze Packet-in message arrival process in its control plane and packet arrival process in its data plane.Then the

M/M/n/m

and

M/M/1/m

queueing models are respectively applied to depict Packet-in message processing process of its controller clusters and packet processing process of its OpenFlow switches. On this basis

we establish a prioritized queueing model of OpenFlow packet forwarding

and derive packet forwarding delays of different priorities and its cumulative distribution function. Finally

experimental evaluation in virtue of the controller performance measurement tool OFsuite_Performance shows that our proposed

M/M/n/m

model can accurately estimate actual performance of controller clusters compared with existing models. Meanwhile

we contrast packet forwarding delays of different priorities in various cases and their CDF curves by numerical analysis

which provides effective references for practical deployments of software-defined core networks.