This Recommendation specifies the requirements for providing the Inter-IMS network to network interface (NNI). This Recommendation endorses ETSI TS 129 165 V10.21.0 (2016-01)@ Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; Inter-IMS Network to Network Interface (NNI) (3GPP TS 29.165 version 10.21.0 Release 10).