P2P mode

[felinira]

Currently the code seems to send all files via a relay. Would it be within the scope of this project to implement a peer to peer mode? It would probably require some modification to the portal to negotiate some out-of-band channel.

In the best case this negotiation would be encrypted, see #3

I could probably contribute code if this is desired.

[landhb]

It's not currently in scope but I'm open to PRs if you're up for it.

I'm not too familiar with P2P protocols but sounds like it would likely be a substantial protocol change. Especially solving the NAT issue etc.

[landhb]

Good article from Tailscale on NAT traversal: https://tailscale.com/blog/how-nat-traversal-works/

[landhb]

Just some notes:

This may be easier to implement in the client with the new API introduced in v0.4.0.

I'd imagine Portal::handshake() and Portal::outgoing() would still happen via the relay over TCP. And then a new UDP/QUIC stream could be used with Portal::send_file() to actually transfer the file in a P2P manner. Peering info could be exchanged through the relay in the TransferInfo or ConnectMessage structs.