You want to improve reliability over a lossy IPSec tunnel.

For improving reliability over a lossy IPSec tunnel, the fragmentation and fragmentation-mtu parameters should be configured. In scenarios where there might be issues with packet size or an unreliable network, setting the IPsec phase 1 to allow for fragmentation will enable large packets to be broken down, preventing them from being dropped due to size or poor network quality. The fragmentation-mtu specifies the size of the fragments. This is aligned with Fortinet's recommendations for handling IPsec VPN over networks with potential packet loss or size limitations.