In an IaaS cloud the physical infrastructure is controlled by service providers, including its security monitoring aspect. Clients hosting their information system are incited to trust the provider's claim (e.g. infrastructure availability) thanks to the assurance given by Service Level Agreements (SLAs). We aim at extending SLAs to include security monitoring terms. In this paper we describe the challenges to reach this goal, we propose a three-step incremental strategy and we apply the first step of this strategy on the case of network IDS (NIDS) monitoring probes. In this case study we select a relevant metric to describe the performance of an NIDS, that is the metric can figure in an SLA and can be measured to verify that the SLA is respected. In particular we propose an in situ verification method of such a metric on a production NIDS and evaluate experimentally and analytically the proposed method.