It's not a requirement, it's just that it tends toward it naturally due to the asymmetric addressability. As long as there are two or more global addresses available to the public on which to run STUN, UPnP, etc, there will be "competition" but it is immeasurably weak compared to what would be possible with direct (non-NAT) addressability. In an environment without those obstacles, systems are naturally designed in a P2P way - simply from the need for scalability.

Case in point A: Skype leveraged an initial P2P design at a time where direct addressability was the norm (and there were many freeware alternatives that allowed direct dialing)... Now that Skype has become dominant, it has switched to a centralized infrastructure 1) because it's owners can (it makes administration, censorship, and surveillance easiest), and 2) because a P2P model no longer makes sense with most users relying on their centralized bootstrap servers.

Case in point B: Dropbox and similar services have replaced self-hosted FTP, I would argue, simply because noone wants to maintain static port mappings and Dropbox is easier.

Even without other incentives, the presence of NAT is a centralizing force that - taken to the extreme (such as with carrier NAT) outright precludes P2P - and that is undesirable. In an Internet with NAT (or any other violation of the end-to-end principal) all systems suffer the same fate: centralization (the antithesis of P2P).

Is your argument that we need to adopt IPv6? Because you'll get no disagreement from me. But something has to be done in the meantime.

I guess I'm going to have to plug my software: http://trustiosity.com/snow

The idea is that it doesn't actually eliminate the horrors of NAT traversal, it just makes it my problem instead of yours.

The current solution is to use other nodes as relays using DHT-style routing and then I put a VM on AWS to bootstrap. The interesting thing is the bootstrap peer is only required for the first connection. Once there is an existing path A-B-C-D, it doesn't matter that zero of them have a public address, you can still use it to send a hole punch message from A to D.

The real problem is that trusting random peers to relay messages allows them to DoS you by filling up the network with Sybils and then not forwarding your messages. So I'm in the process of coming up with solutions to that, probably something along the lines of allowing particular nodes to be designated as trustworthy and preferring those.

Very cool.

Thanks for the link and thanks for taking a stab at a hard problem! Snow looks very promising so far... (I can ping nodes on my LAN over it too, which is usually a sticking point for traversal-oriented software - one is doubly-NAT'd, and there's an SSH server with an Ubuntu banner reachable from afar with UDP packets cutting through both brick walls nicely.) I'm impressed! :)

(FYI, building snow on a fresh Debian Squeeze 686-pae (packages: make g++ libssl-dev libminiupnpc-dev libnatpmp-dev) fails for me at dht.cpp line 220 (ambiguous function call) though; I'll have to read the source more to find the right cast or ::namespace to fix it but it compiles fine on amd64 with an identical set of packages.)

I'll definitely be reading the code more closely!

Thanks.

I can see the bug: The function is overloaded as taking uint64_t or a pointer and I'm passing "0UL" to it, which on 64-bit is an exact match for uint64_t but when 0UL is 32-bit it doesn't know whether to convert it to a uint64_t or a NUL pointer. It probably just needs a cast to uint64_t.