ICMP errors


icmp errors quote the packet they refer to
often truncated, typically 68 bytes quoted

NAT: must rewrite that quoted packet too

quoted packet has a checksum...
if it is truncated, there is no way to verify the checksum anyway
can be ignored
if not truncated, nothing in the real world cares
scapy, which we use for testing, is the only thing I know that does
could just recalculate in that case
cost neglible, packet is tiny