anonymous seeding and public-swarm fallback policies

[synctext]

Tribler should "just work", also when we add anonymity features.
Problem: only a few users will use anonymous seeding initially.

We will (soon) offer a fully anonymous "perfect dark" downloads from anonymous sources using introduction points, rendezvous tunnels, and end-to-end encryption. Traditional unsafe seeding will be disabled by default, in favor of "no unprotected public seeding".

However, our anonymous seeding method will have zero or only a few seeders in the beginning. This bootstrap problem killed a lot of other projects. Without a smooth migration path "perfect dark" downloads are not realistic. A lot of users will get disappointed if they try to download stuff, but nothing works due to the lack of anonymous seeders.

Anonymous downloading can be done from either: unprotected public seeders and hidden seeders. The downloader is protected using various layers of proxies. When the "perfect dark" option fails we need a fallback option: anonymous downloading from public seeds.
In case an anonymous download from anonymous sources fails to make any progress after 180 seconds of trying, the fallback option will kick in and activate proxied downloading using exit nodes from public seeders. The fallback option will be enabled by default and can easily be disabled.

[Arceliar]

I'm not sure I understand the intended use cases for anonymous downloads with a fallback. If you need anonymity, you'd want to disable fallback... and if you don't need anonymity, then you'd probably see performance gains from downloading publicly. The main use case I can think of is to generate as much anonymous cover traffic as possible (for other users to hide in) without impacting one's ability to download. I think that's a good enough reason to do it, but I'm curious if there was something else in mind...

[synctext]

@Arceliar Thank you very much for these comments. We're working on the code right now, this ticket is unclear and use-case is not properly specified. I'll edit the ticket+add your cover traffic argument.

[synctext]

@Arceliar finally polished the text of this ticket.

[Arceliar]

@synctext thanks, the new text makes things much clearer.

[synctext]

Clarification: we now have exit nodes. They can download existing public Bittorrent swarms. Through exit nodes this content becomes available in the encrypted "perfect dark" space.