Network as a Service (NaaS) is recently becoming increasingly relevant, as it will enable cloud providers to select on-demand network services according to application needs. Currently, the lack of QoS control over cloud networking services may significantly impact the application performance. Therefore, two important issues should be addressed by the NaaS providers: (i) determining the QoS to be provided by each cloud networking service, (ii) supporting advanced services differentiation to ensure application flow requirements. In this paper, we present a modeling of QoS-aware cloud networking services and we propose a class of service (CoS) classification to deal with flow cloud application requirements.