What is the difference between ICMP type 3 codes 0,1 and 6,7?

Hi guys,

I am working on an SDN project and I trying to figure out whether to send an ICMP type 3 code 0 or 6 back to host when a network doesn’t exist or route is not in the table, and a code 1 or 7 when a host doesn’t exist or isn’t in the ARP table. I tried researching however I am unclear on the difference between unreachable and unknown and reading RFC 792 and 1122 did not reveal much to me.

If anyone is willing to share their knowledge it would be much appreciated.