Remove uPNP from those home user nat boxes and see how well the nat to
nat connections work. Office firewalls often are heavily restrictive,
use proxy layers to deal with connectivity issues and tend to have less
typical types of traffic.
Considering how many end-users sit behind NAT firewalls and non-firewall
gateways at home, at work, and at public access points all day without
issue, this is a particularly good example of the IETF's ongoing issues
with design-by-committee, particularly committees short on security
engineering and long on special interest. While LECs and ISPs may or may
not feel some pain from LSN, they're equally sure feel better after
crying all the way to the bank.