If a seed does not have an open port, a potential leech with an open port shares their IP & port with the tracker. The seed regularly asks the tracker for potential leeches. If the tracker provides a leech with an open port, then the seed connects to the leeches open port. This connection then allows the leech to download from the seed.

If neither of seed and leech has an open port, no connection can be established and thus no torrenting is possible.

For uTP/UDP:

If both peers (seed & leech) have no open ports, the tracker can use UDP hole punching to temporarily open up a port for the peers. The second peer can then connect directly to the first peer's port which has been opened up by the tracker.

This only works for public torrents and with PEX enabled. For private trackers an open port is required.

[–] JamonBear@sh.itjust.works 2 points 1 day ago

This only works for public torrents and with PEX enabled. For private trackers an open port is required.

Wrong, tracker provide IP and port of leechers, which is just what is needed to hole-punch. It is just slower, as the seeder fetch tracker info 1 to 4 times per hour, so by the time the hole-punched connection is established, leecher has already sucked up all the data from peers with open ports.

If you want to build up ratio on a private tracker with closed port, seeding a piece of a fat 100Gb+ torrent will do.

[–] SincerityIsCool@lemmy.ca 2 points 2 days ago

Thanks for explaining. There is a lot of conflicting information out here - do you have any sources for where I can learn more?