_CHILD_SA: this exchange concerns the establishment of the ESP/AH security association; - INFORMATIONAL: this exchange concerns event notification. Each type of exchange imposes a certain number of required blocks composing the message and defines optional blocks. Notation Designation Security Association Key Exchange Identification - initiator Identification - responder Certificate CERTREQ Certificate Request Authentication Nonce - initiator Nonce - responder Notification Delete Vendor ID Traffic Selector - initiator Traffic Selector - responder Encrypted and Authenticated Configuration Extensible Authentication Table 7.1. Block types 134 Wi-Fi Integration to the 4G Mobile Network Flags: this field includes the following three flags: - R (Response): this flag, positioned at ONE, indicates that this message is a response. An IKE termination must not respond to a response except when authentication has failed; - V (Version): this flag, positioned at ONE, indicates that the IKE termination is able to process a version higher than the one shown in the Major Version field; - I (initiator): this flag, positioned at ONE, indicates that the message is generated by the initiator of the IKE security association. Message ID: this field, coded on four bytes, is an identifier used to control the retransmission of lost messages and to correlate the request and response. It also protects against replay attacks. Length: this field, coded on four bytes, includes the IKE message size. 7.1.2.2. Blocks Each block starts with a generic header containing the Next Payload field, the C (Critical) bit and the Payload Length field (Figure 7.6). Next Payload |C| Reserved Payload Length Figure 7.6. Format of generic block header The Next Payload field indicates the type of block that comes next, thus enabling chaining. The C bit determines the process to be executed when the receiver does not recognize the block: - if the C bit is positioned at ONE, the receiver rejects the message; - if the C bit is positioned at ZERO, the receiver ignore