secunet Security Networks AG secunet Security Networks AG codelabs GmbH This document specifies an extension to the Internet Key Exchange Protocol Version 2 (IKEv2) that enables acknowledgment of IKEv2 message fragments over UDP. The mechanism allows an IKE peer to confirm reception of individual fragments during the IKE_AUTH exchange and any subsequent exchanges where IKEv2 Fragmentation is used. Support for this feature is negotiated using a new Notify Message Status Type during IKE_SA_INIT, and fragment acknowledgments are exchanged using a separate Notification payload. This extension improves reliability when large IKE messages are exchanged, such as those containing post-quantum cryptography (PQC) payloads, and reduces retransmission overhead, thereby improving IKEv2 round-trip times in lossy network conditions.