The Software Defined Networking (SDN) approach has numerous advantages, including the ability to program the network through simple abstractions, provide a centralized view of network state, and respond to changing network conditions. One of the main challenges in designing SDN enabled switches is efficient packet classification in the data plane. As the complexity of SDN applications increases, the data plane becomes more susceptible to Denial of Service (DoS) attacks, which can result in increased delays and packet loss. Accordingly, there is a strong need for network architectures that operate efficiently in the presence of malicious traffic. In particular, there is a need to protect authorized flows from DoS attacks.In this work we utilize a probabilistic data structure to pre-classify traffic with the aim of decoupling likely legitimate traffic from malicious traffic by leveraging the locality of packet flows. We validate our approach by examining a fundamental SDN application: software defined network firewall. For this application, our architecture dramatically reduces the impact of unknown/malicious flows on established/legitimate flows. We explore the effect of stochastic pre-classification in prioritizing data plane classification. We show how pre-classification can be used to increase the effective Quality of Service (QoS) for established flows and reduce the impact of adversarial traffic.