In IKEv2, which exchange establishes the first CHILD_SA?

In the IKEv2 protocol, the initial negotiation consists of two exchanges: IKESAINIT and IKEAUTH. The IKESAINIT exchange establishes a secure channel (the IKE SA) by negotiating cryptographic parameters and performing a Diffie-Hellman key exchange. Following this, the IKEAUTH exchange authenticates the peers and, critically, negotiates and establishes the very first CHILDSA. This first CHILDSA is the IPsec security association that protects the actual data traffic. Subsequent CHILDSAs are created using the CREATECHILDSA exchange.

A. The IKESAINIT exchange only establishes the parent IKE SA (the control channel); it does not create any CHILDSAs for data traffic.
B. The INFORMATIONAL exchange is used for control messages, such as error reporting or deleting SAs, not for creating them.
C. The CREATECHILDSA exchange is used to create additional CHILDSAs or to rekey existing ones after the initial IKEAUTH exchange is complete.

1. FortiOS 7.6.0 IPsec VPN Administration Guide, "IKEv2 protocol" section. This guide details the two initial exchanges, specifying that the first CHILDSA is created during the IKEAUTH exchange.

2. RFC 7296, "Internet Key Exchange Protocol Version 2 (IKEv2)", Section 1.2. This official standard states that the IKEAUTH exchange is used to "create the first CHILDSA".