In the few last years, network virtualization concept has attracted a great deal of interest from both industry and research communities as an important enabler for designing the future Internet architecture. It has emerged as a powerful way to deploy different network architectures and protocols over a shared physical infrastructure. A major challenge associated with the deployment of network virtualization in operator infrastructure is how to describe efficiently virtual resources needed for one or many virtualization services. This paper proposes a virtual resources provisioning schema (VN-SLA) that specifies virtual resources properties and associations. Besides, it regulates agreements and interactions between virtualization network actors. We give the specification model of the proposed VN-SLA schema and we illustrate it with an example.