| What if the lower protocol doesn't have a header that it
| at least 8 bytes? :-)
| 
| I think that's yet another reason why the check and counter bump were
| originally down in the per-protocol hander.
| 
The only reason I can think of is RFC 1122, 3.2.2:
 "Every ICMP error message includes the Internet header and at
  least the first 8 data octets of the datagram that triggered
  the error; more than 8 octets MAY be sent [...]"

But for ICMPv6 the requirement is less strict (RFC 4443, 2.4):
 "Every ICMPv6 error message (type < 128) MUST include as much of
  the IPv6 offending (invoking) packet (the packet that caused the
  error) as possible without making the error message packet exceed
  the minimum IPv6 MTU [IPv6]."