ICMP_TOOBIG_SIZE
ICMP_TOOBIG_SIZE + headroom - sizeof(struct ethhdr));
if (pckt_size > max(max_pcktsz, ICMP_TOOBIG_SIZE)) {
offset = pckt_size - ICMP_TOOBIG_SIZE;
if (data + (ICMP_TOOBIG_SIZE + headroom) > data_end)