Hello,
FCB is a data structure used to reassembey and forward fragments. Every fragmented packet has same IP addresses and same IPID and based on this FCB is able to reassemble the fragments into one packet
when it receives them. FCB has a timeout of 3 seconds and if all fragments of a packets are not received by FCB in 3 seconds, fragment will age out and will get dropped with the message that you see.
Is there any pattern with source IP that you see?
Are the source IP addresses unknown?
Regards,
Rushi