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It’s true that most clients do not support a zero upload configuration, but it’s not inherent to the protocol, and modified clients exist.

I’m not aware of any clients that will refuse to share data with clients that are configured to not upload. I don’t even see how they could determine that, especially in situations where there are no other peers to upload to, and given that stats are entirely self-reported and clients that send bogus numbers exist.

You would need a central tracker that cares, which is what private torrent communities rely on, but not public/DHT torrents such as those discussed here.

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You’re correct about seeds, but peers who are also downloading will often stop sharing with you if you stop sharing with them. Seeds generally are configured to try to give different pieces to different peers so that they can send them to each other and reduce load on the seed; they don’t want to give you the entire file directly unless you’re the only person downloading. And peers prioritize and filter which other ones they’ll send pieces to based on reciprocity.

You will probably get the data eventually, and it really depends on the composition and configuration of the swarm, but generally, you do need to upload if you want to ensure the fastest and most reliable download.

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Long-running torrents are mostly populated by seeders. Bit torrent was originally designed for a lot of downloaders to get a file at the same time with limited seeding bandwidth, so leechers would need to trade with each other a lot, but that's not really the situation most torrents are in today.
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It depends what you’re downloading. Peer prioritization mechanics have been relevant for a lot of my recent downloads.
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You can, but you will slow down your own downloads dramatically by doing so. In some cases you will fail to finish them.

The case for doing this would be just so you can have this ridiculous legal defence Meta seem to be trying to pull out. Really no other good reason. Even for the most parasitic leeches, zero upload is a bad strategy.

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Seeders don't know how much data you shared with other leechers.
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Yes. So?
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So you can download from seeders as fast as they can upload.
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