A Network Service Descriptor (NSD) is generated, from Virtualized Network Functions (VNFs) existing in a VNF catalog, for instantiating a network service which satisfies given network service requirements. From the VNF catalog, a system selects VNFs that provide functionalities and architectural blocks required to fulfill the NS requirements. The system generates at least a VNF forwarding graph (VNFFG), which includes relations between the functionalities and the architectural blocks of the selected VNFs as well as traffic flows between the selected VNFs. The system then creates the NSD, which includes the VNFFG, for instantiating the network service.