I was having some weird times with our Virtual Fortigate Appliance.
It had a very steady IPSEC VPN connection with our of our sites but for some reason the packets were being dropped with no good reason although the tunnel remained connected.
I found the following in the logs:
“Received ESP packet with unknown SPI”.
Official Fortigate KBs claim turning on DPD should prevent this from happening.
But in actuality it did NOT.
My intuition somewhat told me that this has got something to do with PFS as it deals with generating keys per data. I was right. After disabling it the tunnel became stable like a rock.