As someone who works on P2P applications, if all I had to do was use PCP to open a pinhole in a firewall it would still be a big improvement over dealing with NAT.
For one thing lack of NAT makes it much easier to deal with multi-homed systems. For various reasons, multi-home is much more common with IPv6 than IPv4. Without NAT I can discover thing like what an address's scope is without querying the network. Having to ask a remote server to discover a global address turns what should be an atomic operation into a potentially troublesome state machine.
NAT also create annoying corner cases when there are local peers reachable via an address which also has a NATed global address. You may not be able to tell that the peer's address is not globally reachable, which is a problem if you want to advertise that peer to others.
> For one thing lack of NAT makes it much easier to deal with multi-homed systems.
It’s the opposite imo. Lack of NAT makes it impossible to do policy based routing enforced at a router level, eg route VoIP over ISP 1, and Web over ISP 2. Without NAT, each IPv6 PC is issued one or more IP addresses per WAN, but has no idea when it’s appropriate to use one over the other. (SLAAC router advertisements aren’t sophisticated enough)
This is the sort of thing I was talking about when I mentioned negative externalities elsewhere in this thread. NAT is nice if you want to do things like that, but those sorts of tricks create big problems for people like me. Being behind NAT with multiple external IPs and an unknown policy for which gets used when is a nightmare scenario for P2P applications.
For one thing lack of NAT makes it much easier to deal with multi-homed systems. For various reasons, multi-home is much more common with IPv6 than IPv4. Without NAT I can discover thing like what an address's scope is without querying the network. Having to ask a remote server to discover a global address turns what should be an atomic operation into a potentially troublesome state machine.
NAT also create annoying corner cases when there are local peers reachable via an address which also has a NATed global address. You may not be able to tell that the peer's address is not globally reachable, which is a problem if you want to advertise that peer to others.