Stateful NAT: Improve ICMP support in allocator #995
Description
Right now, the stateful NAT allocator reuses the TCP/UDP ports fields for allocating values for ICMP identifiers. It would be better to have a cleaner design, and only allocate a single value in the case of ICMP.
Another question about ICMP, although not related to the allocator, is how we should translate (if at all) packets such as ICMP "Destination Error" messages when they do not have any inner packet (right now we translate ICMP Query messages, and ICMP Error messages with inner packets - should we do something particular for other packets?)