It's just a dumb mistake. All it takes is a "next layer header length" field. It would have been very simple. You don't even really need that, and as proof, ta

It's just a dumb mistake. All it takes is a "next layer header length" field. It would have been very simple. You don't even really need that, and as proof, take ICMP ... which was designed as part of IP ... actually does do this. Routers are already required to copy and include the header of the packet that triggered an ICMP error.