You need to be able to accept incoming connections to be able to fully participate in the network. Last time I seriously looked into this, BitTorrent clients didn't support any sort of NAT hole punching (and they often work over TCP in any case). Try running a client with and without a forwarded port and you will see massive difference in the number of peer connections.

So you're saying there's a chance

Of course, but if everyone is behind the NAT then no one in the swarm can connect to any one. If this is a popular torrent when someone with the connectivity would show up, eventually, but otherwise good luck. Recently it took me four months to complete one torrent and I was the one with the real IP.

> NAT hole punching

Could we just throw a STUN service in front of this, then?

I think I might be doing that already, as this is the first I've heard of this. Unless Mullvad was automatically opening a port for me.

Is it possible a lot of average torrenters are already not port forwarding?

Transmission has supported UPnP and NAT-PMP for many years. Although it doesn't always work as reliably as having a client with directly routable address(es), it does exist and works okay.

Because a least one person has to have forwarded ports for them to form a direct connection. [0]

This will degrade torrent performance and make torrenting worse, routers normally have uPnP enabled these days so we forget about it, but this will make it so you can’t connect to any other users who are also using Mullvad, for one.

[0]https://superuser.com/questions/1053414/how-does-port-forwar...

> routers normally have uPnP enabled these days

From what I understand, uPnP took off for a while, but started to become much less common about a decade ago because of the security issues it caused. I think most routers come with it disabled by default now. (If you know of any surveys indicating otherwise, I'd be curious to read them.)

Part of it is that hole punching became a standard feature for new protocols, so the need to forward ports has been reduced.

Most consumer routers I've seen come with UPnP on while SOHO routers require explicit configuration

In order to download a file via Torrent, someone has to upload it, and when using Torrent via VPN, the file cannot be uploaded without port forwarding.

Actually, the initial seeder with a closed port can upload if someone else has an open port. Generally a lack of port forwarding means you can only connect to others who do have port forwarding.

Uploading can still happen even without open ports. The open port part is that someone has to initiate the connection after the connection is established anyone can send anything in any direction.