Which exchange lakes care of DoS protection in IKEv2?

IKE_SA_INIT Exchange:

The IKE_SA_INIT exchange is the first step in the IKEv2 negotiation process. It is responsible for setting up the initial security association (SA) and performing Diffie-Hellman key exchange.

During this exchange, the responder may employ various measures to protect against Denial of Service (DoS) attacks, such as rate limiting and the use of puzzles to increase the computational cost for an attacker.

DoS Protection Mechanisms:

One key method involves limiting the number of half-open SAs from any single IP address or subnet.

The IKE_SA_INIT exchange can also incorporate the use of stateless cookies, which help to verify the initiator's legitimacy without requiring extensive resource allocation by the responder until the initiator is verified.

References:

RFC 5996: Internet Key Exchange Protocol Version 2 (IKEv2)​(RFC Editor)​.

RFC 8019: Protecting Internet Key Exchange Protocol Version 2 (IKEv2) Implementations from Distributed Denial-of-Service Attacks​(IETF Datatracker)​.