Section: 13.3.1.1 Direct communication
Content: NRF and NF shall authenticate each other during discovery, registration, and access token request. In direct communication, NF and NRF shall use one of the following methods for authentication: - If the PLMN uses protection at the transport layer as described in clause 13.1, authentication provided by the transport layer protection solution shall be used for mutual authentication of the NRF and NF. - If the PLMN does not use protection at the transport layer, mutual authentication of NRF and NF may be implicit by NDS/IP or physical security (see clause 13.1).