Most Internet Key Exchange (IKEv2) messages are usually small. IP defines a mechanism for fragmentation of oversized UDP messages, but implementations vary in the maximum message size supported. Some NAT and/or Firewall implementations and intermittent routers do not handle IP fragments and these fragmented packets might be dropped. This can cause issues when large IKEv2 messages like digital certificates are transferred over the network.
With this feature, IPSec can fragment large messages at IKEv2 itself and replace them with a series of smaller messages as defined in RFC 7383. The IP datagrams are small enough that fragmentation does not occur at the IP level.