Multiple Key Exchanges in IKEv2

This document describes how to extend the Internet Exchange Protocol Version 2 (IKEv2) to allow multiple key exchanges to take place while computing of a shared secret during a Security Association (SA) setup. The primary application of this feature in IKEv2 is the ability to perform one or more post-quantum key exchanges in conjunction with the classical (Elliptic Curve) Diffie- Hellman key exchange, so that the resulting shared key is resistant against quantum computer attacks. Another possible application is the ability to combine several key exchanges in situations when no single key exchange algorithm is trusted by both initiator and responder. This document updates RFC7296 by renaming a tranform type 4 from Diffie- Hellman Group (D-H) to Key Exchange Method (KE) and renaming a field in the Exchange Payload from Diffie-Hellman Group Num to Key Exchange Method. It also renames an IANA registry for this transform type from Transform Type 4 - Diffie- Hellman Group Transform IDs to Transform Type 4 - Exchange Method Transform IDs. These changes generalize key exchange algorithms that can be used in IKEv2.